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

[mikecase (-- gone --), 2016/06/01 21:59:20]

nit: everywhere s/emma/EMMA

[BigBossZhiling, 2016/06/01 23:36:24]

Done.

+
+In order to use emma code coverage, we need to create build time .em file and run time

[mikecase (-- gone --), 2016/06/01 21:59:20]

nit: s/run time/runtime

[BigBossZhiling, 2016/06/01 23:36:24]

Done.

+.ec file. Then we need to merge them to become a readable html file.

[mikecase (-- gone --), 2016/06/01 21:59:20]

nit: everywhere............

s/.ec/**.ec**
s/.em/**.em**

to make it easier to read.


Also, instead of using the word "merge" you might want to replace it with
something like "process then using the build/android/generate_emma_html.py
script"

[BigBossZhiling, 2016/06/01 23:36:24]

Done.

+
+## Steps to make use of code coverage

[mikecase (-- gone --), 2016/06/01 21:59:20]

nit: s/Steps to make use of code coverage/How to collect EMMA coverage data

[BigBossZhiling, 2016/06/01 23:36:24]

Done.

+
+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 create:

[mikecase (-- gone --), 2016/06/01 21:59:20]

nit: s/create/created

[BigBossZhiling, 2016/06/01 23:36:24]

Done.

+   `EMMA: writing [html] report to
+   [<your_current_directory>/example.html] …`