Android: add documentation about junit tests.

docs/android_test_instructions.md

 # Android Test Instructions
 
 Device Setup Tests are runnable on physical devices or emulators. See the
 instructions below for setting up either a physical device or an emulator.
 
 [TOC]
 
 ## Physical Device Setup **ADB Debugging**
 
 In order to allow the ADB to connect to the device, you must enable USB
@@ skipping 102 matching lines @@
 All functional tests are run using `build/android/test_runner.py`.
 Tests are sharded across all attached devices. In order to run tests, call:
 `build/android/test_runner.py <test_type> [options]`
 For a list of valid test types, see `test_runner.py --help`. For
 help on a specific test type, run `test_runner.py <test_type> --help`.
 
 The commands used by the buildbots are printed in the logs. Look at
 http://build.chromium.org/ to duplicate the same test command as a particular
 builder.
 
 If you build in an output directory other than "out", you may have to tell

[dgn, 2015/10/26 10:12:05]

nit: s/"out"/`out` and for a few other file names in the paragraph

 test\_runner.py where you place it. Say you build your android code in
 out\_android, then do `export CHROMIUM_OUT_DIR=out_android` before running the
 command below. You have to do this even if your "out" directory is a symlink
-pointing to "out_android".
+pointing to "out_android". You can also use ```--output-directory``` to point
+to the path of your output directory, for example,
+```--output-directory=out_android/Debug```.
 
 ## INSTALL\_FAILED\_CONTAINER\_ERROR or INSTALL\_FAILED\_INSUFFICIENT\_STORAGE
 
 If you see this error when test\_runner.py is attempting to deploy the test
 binaries to the AVD emulator, you may need to resize your userdata partition
 with the following commands:
 
 ```shell
 # Resize userdata partition to be 1G
 resize2fs android_emulator_sdk/sdk/system-images/android-19/x86/userdata.img 1G
 
 # Set filesystem parameter to continue on errors; Android doesn't like some
 # things e2fsprogs does.
 tune2fs -e continue android_emulator_sdk/sdk/system-images/android-19/x86/userdata.img
 ```
 
 ## Symbolizing Crashes
 
 Crash stacks are logged and can be viewed using adb logcat. To symbolize the
 traces, pipe the output through
 `third_party/android_platform/development/scripts/stack`. If you build in an
 output directory other than "out", pass
 `--chrome-symbols-dir=out_directory/{Debug,Release}/lib` to the script as well.
 
+## JUnit tests
+
+JUnit tests are Java unittests running on the host instead of the target device.
+They are faster to run and therefore are recommended over instrumentation tests
+when possible.
+
+The JUnits tests are usually following the pattern of *target*\_junit\_tests,
+for example, ```content_junit_tests``` and ```chrome_junit_tests```.
+
+When adding a new JUnit test, the associated ```BUILD.gn``` file must be updated
+. For example, adding a test to ```chrome_junit_tests``` requires to update

[Bernhard Bauer, 2015/10/23 15:20:21]

Does this work, or do you get a space before the period?

[mlamouri (slow - plz ping), 2015/10/26 11:12:18]

It works.

+```chrome/android/BUILD.gn```. If you are a GYP user, you will not need to do
+that step in order to run the test locally but it is still required for GN users
+to run the test.
+
+```shell
+# Build the test suite.
+ninja -C out/Release chrome_junit_tests

[dgn, 2015/10/26 10:12:05]

Why do you demo it with a release build? Aren't we supposed to do dev work on
Debug?

[mlamouri (slow - plz ping), 2015/10/26 11:12:18]

Consistency with the rest of the documentation. We can change the entire
document if needed.

+
+# Run the test suite.
+build/android/test_runner.py junit -s chrome_junit_tests --release -vvv
+
+# Run a subset of tests. You might need to pass the package name for some tests.
+build/android/test_runner.py junit -s chrome_junit_tests --release -vvv
+-f "org.chromium.chrome.browser.media.*"
+```
+
 ## Gtests
 
 ```shell
 # Build a test suite
 ninja -C out/Release content_unittests_apk
 
 # Run a test suite
 build/android/test_runner.py gtest -s content_unittests --release -vvv
 
 # Run a subset of tests
@@ skipping 93 matching lines @@
 See
 https://sites.google.com/a/chromium.org/dev/developers/testing/webkit-layout-tests
 
 ## Running GPU tests
 
 (e.g. the "Android Debug (Nexus 7)" bot on the chromium.gpu waterfall)
 
 See http://www.chromium.org/developers/testing/gpu-testing for details. Use
 --browser=android-content-shell. Examine the stdio from the test invocation on
 the bots to see arguments to pass to src/content/test/gpu/run\_gpu\_test.py.