Added instructions on how to use code coverage.

build/android/docs/coverage.md

+# 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 run time
+.ec file. Then we need to merge them to become a readable html file.
+
+## Steps to make use of code coverage
+
+1. Make sure that 'emma_coverage = true' is in your gn args.

[mikecase (-- gone --), 2016/06/01 20:28:01]

Should these be ` instead of '?

[BigBossZhiling, 2016/06/01 21:22:43]

Done.

+2. Build the apk. (With correct gn args, this will create the build time .em
+   file)

[mikecase (-- gone --), 2016/06/01 20:28:01]

Can probably combine the first steps into something like this.


1. Build your APK with the GN arg emma_coverage=true.
  ```
  gn args out/Debug
  > target_os = "android"
  > emma_coverage = true
  ```

[BigBossZhiling, 2016/06/01 21:22:43]

Done.

+3. 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'.
+4. 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 create:
+   'EMMA: writing [html] report to
+   [<your_current_directory>/example.html] …'