[Coverage] Add documentation for Clang source based code coverage.

Change-Id: I6c3920ca92cf05b41fe69c74936cc0578841847f

Bug: chromium:821851
Change-Id: I6c3920ca92cf05b41fe69c74936cc0578841847f
Reviewed-on: https://chromium-review.googlesource.com/952296
Commit-Queue: Yuke Liao <liaoyuke@chromium.org>
Reviewed-by: Max Moroz <mmoroz@chromium.org>
Cr-Commit-Position: refs/heads/master@{#543125}

docs/code_coverage.md

+# Code coverage in Chromium
+
+## Coverage script.
+[Coverage script] helps to generate clang source-based code coverage report
+locally and make it easy to identify under-covered areas.
+
+Currently, this script supports running on Linux, Mac, iOS and ChromeOS
+platforms, and supports displaying code coverage results per directory, per
+component and per file.
+
+In order to generate code coverage report, you need to first add
+`use_clang_coverage=true` and `is_component_build=false` GN flags to args.gn
+file in your build output directory (e.g. out/coverage).
+
+Existing implementation requires `is_component_build=false` flag
+because coverage info for dynamic libraries may be missing and
+`is_component_build` is set to true by `is_debug` unless it is explicitly set
+to false.
+
+Example usage:
+```
+gn gen out/coverage --args='use_clang_coverage=true is_component_build=false'
+gclient runhooks
+python tools/code_coverage/coverage.py crypto_unittests url_unittests
+      -b out/coverage -o out/report -c 'out/coverage/crypto_unittests'
+      -c 'out/coverage/url_unittests --gtest_filter=URLParser.PathURL'
+      -f url/ -f crypto/
+```
+
+For more options, please refer to the script.
+
+## Reporting problems
+
+For any breakage report and feature requests, please [file a bug].
+
+[Coverage script]: https://cs.chromium.org/chromium/src/tools/code_coverage/coverage.py
+[file a bug]: https://bugs.chromium.org/p/chromium/issues/entry?components=Tools%3ECodeCoverage
\ No newline at end of file

tools/code_coverage/coverage.py

@@ -11,10 +11,9 @@
   "use_clang_coverage=true" and "is_component_build=false" GN flags to args.gn
   file in your build output directory (e.g. out/coverage).
 
-  Clang Source-based Code Coverage requires "is_component_build=false" flag
-  because: There will be no coverage info for libraries in component builds and
-  "is_component_build" is set to true by "is_debug" unless it is explicitly set
-  to false.
+  Existing implementation requires "is_component_build=false" flag because
+  coverage info for dynamic libraries may be missing and "is_component_build"
+  is set to true by "is_debug" unless it is explicitly set to false.
 
   Example usage: