how_to_add_your_feature_flag.md

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
• content/public/common/content_features.h

If you want to use the flag in blink, you should also read Runtime Enable Features.

You can refer to this CL to see

1. where to add the base::Feature [1] [2]
2. how to use it [1]
3. how to wire the base::Feature to WebRuntimeFeatures [1] [2] [3] [4]
4. how to use it in blink [1]

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
• chrome/browser/flag_descriptions.cc
• chrome/browser/flag_descriptions.h
• 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.

# 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 as an example.