Update EMMA code coverage documentation.

Review-Url: https://codereview.chromium.org/2426583004
Cr-Commit-Position: refs/heads/master@{#425925}

build/android/docs/coverage.md

@@ -1,32 +1,42 @@
 # Android code coverage instructions
 
-This is instruction for code coverage for android instrumentation and junit tests.
+These are instructions for collecting code coverage data 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
+In order to use EMMA code coverage, we need to create build time **.em** files
+and runtime **.ec** files. 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.
+1. Use the following GN build arguments:
    ```
-   gn args out-gn/Debug
-   > target_os = "android"
-   > emma_coverage = true
+   target_os = "android"
+   emma_coverage = true
+   emma_filter = "org.chromium.chrome.browser.ntp.*,-*Test*,-*Fake*,-*Mock*"
    ```
-   By doing so, **.em** files will be created in out-gn/Debug.
+   The filter syntax is as documented for the [EMMA coverage
+   filters](http://emma.sourceforge.net/reference/ch02s06s02.html).
+
+   Now when building, **.em** files will be created in the build directory.
 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:
+   `out/Debug/bin/run_chrome_junit_tests --coverage-dir /tmp/coverage`.
+3. Turn off strict mode when running instrumentation tests by adding
+   `--strict-mode=off` because the EMMA code causes strict mode violations by
+   accessing disk.
+4. Use a pre-L Android OS (running Dalvik) because code coverage is not
+   supported in ART.
+5. The coverage results of junit and instrumentation tests will be merged
+   automatically if they are in the same directory.
+6. Now we have both .em and .ec files. We can create a html report using
+   `generate_emma_html.py`, for example:
    `build/android/generate_emma_html.py --coverage-dir /tmp/coverage/
-   --metadata-dir out-gn/Debug/ --output example.html`.
+   --metadata-dir out/Debug/ --output example.html`.
    Then an example.html containing coverage info will be created:
    `EMMA: writing [html] report to
    [<your_current_directory>/example.html] …`