Check in documentation for WPT import/export to //docs.

Copied from:
https://sites.google.com/a/chromium.org/dev/blink/importing-the-w3c-tests
and then edited and updated.

BUG=665494

Review-Url: https://codereview.chromium.org/2660083002
Cr-Commit-Position: refs/heads/master@{#446964}

docs/testing/web_platform_tests.md

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+# Web Platform Tests
+
+The Web Platform Tests project provides a large number of conformance tests for
+different aspects of the Web Platform. Currently the tests are hosted on GitHub.
+There are two main repositories, one for the CSS Working Group (aka csswg-test),
+and one for pretty much everything else (aka web-platform-tests).
+
+There is a plan to merge csswg-test into web-platform-tests, so later, there
+will be only one repository.
+
+[TOC]
+
+## Import
+
+Chromium has mirrors
+([web-platform-tests](https://chromium.googlesource.com/external/w3c/web-platform-tests/),
+[csswg-test](https://chromium.googlesource.com/external/w3c/csswg-test/)) of the
+GitHub repos, and periodically imports a subset of the tests so that they are
+run as part of the regular Blink layout test testing process.
+
+The goal of this process are to be able to run the Web Platform Tests unmodified
+locally just as easily as we can run the Blink tests, and ensure that we are
+tracking tip-of-tree in the Web Platform Tests repository as closely as
+possible, and running as many of the tests as possible.
+
+### Automatic import process
+
+There is an automatic process for updating the Chromium copy of the Web Platform
+Tests. The import is done by the builder [w3c-test-autoroller
+builder](https://build.chromium.org/p/chromium.infra.cron/builders/w3c-test-autoroller).
+
+The easiest way to check the status of recent imports is to look at:
+
+-   Recent logs on Buildbot for [w3c-test-autoroller
+    builder](https://build.chromium.org/p/chromium.infra.cron/builders/w3c-test-autoroller)
+-   Recent CLs created by
+    [blink-w3c-test-autoroller@chromium.org](https://codereview.chromium.org/search?owner=blink-w3c-test-autoroller%40chromium.org).
+
+### Skipped tests
+
+We control which tests are imported via a file called
+[W3CImportExpectations](../../third_party/WebKit/LayoutTests/W3CImportExpectations),
+which has a list of directories to skip while importing.
+
+In addition to the directories and tests explicitly skipped there, tests may
+also be skipped for a couple other reasons, e.g. if the file path is too long
+for Windows. To check what files are skipped in import, check the recent logs
+for [w3c-test-autoroller
+builder](https://build.chromium.org/p/chromium.infra.cron/builders/w3c-test-autoroller).
+
+### Manual import
+
+To pull the latest versions of the tests that are currently being imported, you
+can also directly invoke the
+[wpt-import](../../third_party/WebKit/Tools/Scripts/wpt-import) script.
+
+That script will pull the latest version of the tests from our mirrors of the
+upstream repositories. If any new versions of tests are found, they will be
+committed locally to your local repository. You may then upload the changes.
+
+If you wish to add more tests (by un-skipping some of the directories currently
+skipped in `W3CImportExpectations`), you can modify that file locally and commit
+it, and on the next auto-import, the new tests should be imported. If you want
+to import immediately, you can also run `wpt-import --allow-local-commits`.
+
+## Contributing tests back to the Web Platform Tests project.
+
+If you need to make changes to Web Platform Tests, just commit your changes
+directly to
+[LayoutTests/external/wpt](../../third_party/WebKit/LayoutTests/external/wpt)
+and the changes will be automatically upstreamed within 24 hours.
+
+Note: if you're adding a new test in `external/wpt`, you'll need to re-generate
+MANIFEST.json manually until [CL 2644783003](https://crrev.com/2644783003) is
+landed. The command to do so is:
+
+```bash
+Tools/Scripts/webkitpy/thirdparty/wpt/wpt/manifest --work \
+    --tests-root=LayoutTests/external/wpt
+```
+
+### Will the exported commits be linked to my GitHub profile?
+
+The email you commit with in Chromium will be the author of the commit on
+GitHub. You can [add it as a secondary address on your GitHub
+account](https://help.github.com/articles/adding-an-email-address-to-your-github-account/)
+to link your exported commits to your GitHub profile.
+
+### What if there are conflicts?
+
+This cannot be avoided entirely as the two repositories are independent, but
+should be rare with frequent imports and exports. When it does happen, manual
+intervention will be needed and in non-trivial cases you may be asked to help
+resolve the conflict.
+
+### Direct pull requests
+
+It's still possible to make direct pull requests to web-platform-tests. The
+processes for getting new tests committed the W3C repos are at
+[http://testthewebforward.org/docs/]. Some specifics are at
+[http://testthewebforward.org/docs/github-101.html].