Added instructions on how to use code coverage.

build/android/docs/coverage.md

Index: build/android/docs/coverage.md
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+# Android code coverage instructions
+
+This is instruction for code coverage for android instrumentation and junit tests.
+
+[TOC]
+
+## How EMMA coverage works
+
+In order to use EMMA code coverage, we need to create build time **.em** file and runtime
+**.ec** file. Then we need to process them using the
+build/android/generate_emma_html.py script.
+
+## How to collect EMMA coverage data
+
+1. Build your APK with the GN arg emma_coverage=true.
+   ```
+   gn args out-gn/Debug
+   > target_os = "android"
+   > emma_coverage = true
+   ```
+   By doing so, **.em** files will be created in out-gn/Debug.
+2. Run tests, with option `--coverage-dir <directory>`, to specify where to save
+   the .ec file. For example, you can run chrome junit tests:
+   `out-gn/Debug/bin/run_chrome_junit_tests --coverage-dir /tmp/coverage`.
+3. Now we have both .em and .ec files. We can merge them and create a html file,
+   using generate_emma_html.py. For example, generate_emma_html.py can be called
+   this way:
+   `build/android/generate_emma_html.py --coverage-dir /tmp/coverage/
+   --metadata-dir out-gn/Debug/ --output example.html`.
+   Then an example.html containing coverage info will be created:
+   `EMMA: writing [html] report to
+   [<your_current_directory>/example.html] …`