Add a document to explain how to add a feature flag (chrome://flags)

TBR=asvitkine@chromium.org

Bug: N/A
Change-Id: I53bda41c047ed970417df5a5ca308855cbccbde5
Reviewed-on: https://chromium-review.googlesource.com/594754
Commit-Queue: Makoto Shimazu <shimazu@chromium.org>
Reviewed-by: Nico Weber <thakis@chromium.org>
Cr-Commit-Position: refs/heads/master@{#492501}

docs/README.md

@@ -133,6 +133,8 @@ used when committed.
     more performant
 *   [ES6 Support in Chromium](es6_chromium.md) - Implementation of ECMAScript6
     features in Chromium
+*   [Adding a new feature flag in chrome://flags](how_to_add_your_feature_flag.md) - Quick
+    guide to add a new feature flag to experiment your feature.
 
 ### Testing
 *   [Running and Debugging Layout Tests](testing/layout_tests.md)

docs/how_to_add_your_feature_flag.md

@@ -0,0 +1,53 @@
+# Adding a new feature flag in chrome://flags
+
+This document describes how to add your new feature behind a flag.
+
+## Step 1: Adding a new `base::Feature`
+This step would be different between where you want to use the flag.
+For example, if you want to use the flag in src/content, you should add a base::Feature to the following files:
+
+* [content/public/common/content_features.cc](https://cs.chromium.org/chromium/src/content/public/common/content_features.cc)
+* [content/public/common/content_features.h](https://cs.chromium.org/chromium/src/content/public/common/content_features.h)
+
+If you want to use the flag in blink, you should also read
+[Runtime Enable Features](https://www.chromium.org/blink/runtime-enabled-features).
+
+You can refer to [this CL](https://chromium-review.googlesource.com/c/554510/)
+to see
+
+1. where to add the base::Feature
+[[1](https://chromium-review.googlesource.com/c/554510/8/content/public/common/content_features.cc#253)]
+[[2](https://chromium-review.googlesource.com/c/554510/8/content/public/common/content_features.h)]
+2. how to use it
+[[1](https://chromium-review.googlesource.com/c/554510/8/content/common/service_worker/service_worker_utils.cc#153)]
+3. how to wire the base::Feature to WebRuntimeFeatures
+[[1](https://chromium-review.googlesource.com/c/554510/8/content/child/runtime_features.cc)]
+[[2](https://chromium-review.googlesource.com/c/554510/8/third_party/WebKit/public/platform/WebRuntimeFeatures.h)]
+[[3](https://chromium-review.googlesource.com/c/554510/third_party/WebKit/Source/platform/exported/WebRuntimeFeatures.cpp)]
+[[4](https://chromium-review.googlesource.com/c/554510/8/third_party/WebKit/Source/platform/RuntimeEnabledFeatures.json5)]
+4. how to use it in blink
+[[1](https://chromium-review.googlesource.com/c/554510/8/third_party/WebKit/Source/core/workers/WorkerThread.cpp)]
+
+Also, this patch added a virtual test for running layout tests with the flag.
+When you add a flag, you can consider to use that.
+
+## Step 2: Adding the feature flag to the chrome://flags UI.
+You have to modify these four files in total.
+
+* [chrome/browser/about_flags.cc](https://cs.chromium.org/chromium/src/chrome/browser/about_flags.cc)
+* [chrome/browser/flag_descriptions.cc](https://cs.chromium.org/chromium/src/chrome/browser/flag_descriptions.cc)
+* [chrome/browser/flag_descriptions.h](https://cs.chromium.org/chromium/src/chrome/browser/flag_descriptions.h)
+* [tools/metrics/histograms/enums.xml](https://cs.chromium.org/chromium/src/tools/metrics/histograms/enums.xml)
+
+At first you need to add an entry to __about_flags.cc__,
+__flag_descriptions.cc__ and __flag_descriptions.h__. After that, try running the following test.
+
+```bash
+# Build unit_tests
+ninja -C out/Default unit_tests
+# Run AboutFlagsHistogramTest.CheckHistograms
+./out/Default/unit_tests --gtest_filter=AboutFlagsHistogramTest.CheckHistograms
+```
+
+That test will ask you to add several entries to enums.xml.
+You can refer to [this CL](https://chromium-review.googlesource.com/c/593707) as an example.