Add documentation for WebRTC telemetry tests.

Bug: 
Change-Id: Ib7a17c7dd7083ff7020eb4a778cf07f4c9f5b0fa
Reviewed-on: https://chromium-review.googlesource.com/550056
Commit-Queue: Edward Lesmes <ehmaldonado@chromium.org>
Reviewed-by: Henrik Kjellander <kjellander@chromium.org>
Reviewed-by: Ned Nguyen <nednguyen@google.com>
Cr-Commit-Position: refs/heads/master@{#482722}

docs/speed/benchmark_harnesses/webrtc_perf.md

@@ -0,0 +1,86 @@
+# WebRTC Telemetry Tests
+
+[TOC]
+
+## Overview
+
+For its telemetry benchmarks, WebRTC uses the
+[test pages](https://webrtc.github.io/test-pages/) and
+[sample pages](https://webrtc.github.io/samples/) from the
+[WebRTC GitHub project](https://github.com/webrtc).
+
+These are downloaded by the
+[`update_webrtc_cases`](../../../tools/perf/page_sets/update_webrtc_cases)
+script into the
+[`webrtc_cases`](../../../tools/perf/page_sets/webrtc_cases/)
+directory, and then referenced in
+[`webrtc_cases.py`](../../../tools/perf/page_sets/webrtc_cases.py),
+which controls the user interactions and duration of the test.
+
+The [`webrtc.py`](../../../tools/perf/benchmarks/webrtc.py)
+benchmark specifies which metrics should be collected for the
+test pages, and extra options that we pass to the test to fake the real camera
+and skip assing permission to get access to the video and audio from the user.
+
+
+## Running the Tests
+
+To run the tests in the browser you should simply open the page, either in
+GitHub or the `webrtc_cases` directory.
+
+To collect traces and compute metrics you can run the following command
+(assuming you are in `chromium/src`):
+```
+./tools/perf/run_benchmark webrtc --browser-executable=out/Release/chrome
+```
+
+You can filter the pages you want to test using the `--story-tag-filter` flag
+with the tags specified in
+[`webrtc_cases.py`](../../../tools/perf/page_sets/webrtc_cases.py#127).
+
+For example, to run only the *multiple-peerconnections* test page, you can use
+the following command:
+```
+./tools/perf/run_benchmark webrtc --browser-executable=out/Release/chrome
+--story-tag-filter=stress
+```
+
+
+## Adding Telemetry Tests for WebRTC
+
+To add a new test page you should:
+
+1. **Add a new test page to the
+[test-pages](https://github.com/webrtc/test-pages) repository under the
+[`src` directory](https://github.com/webrtc/test-pages/tree/gh-pages/src) in the
+gh-pages branch.**
+
+ The test page should be named `index.html`, and the logic for the test should
+ be inside the `js` folder in a file named `main.js`. Don’t forget to reference
+ it at the
+ [index page](https://github.com/webrtc/test-pages/blob/gh-pages/index.html).
+
+ See the *[multiple-peerconnections](https://github.com/webrtc/test-pages/tree/gh-pages/src/multiple-peerconnections)*
+ test page for an example.
+
+2. **Edit the [`update_webrtc_cases_script`](../../../tools/perf/page_sets/update_webrtc_cases#21)
+to reference the page you added, and run it.**
+
+ This will download the `index.html` file and the `main.js` file into the
+ `webrtc_cases` directory, and rename them with the name of the test.
+
+ For example, the *multiple-peerconnections* test page will be downloaded as
+ `multiple-peerconnections.html` and `multiple-peerconnections.js`.
+
+3. **Add a new class to `webrtc_cases.py` to load the page and control the user
+interactions.**
+
+ See the
+ [`MultiplePeerConnections`](../../../tools/perf/page_sets/webrtc_cases.py#101)
+ class for example.
+
+4. **Add the story to the `WebrtcPageSet` class.**
+
+ See [here](../../../tools/perf/page_sets/webrtc_cases.py#127) for example.
+
+5. **Submit your changes as a CL**