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 106 matching lines @@
 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
 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

[dgn, 2015/10/22 18:09:23]

I think triple backtick make a new paragraph. Did you check how the file is
rendered on gitiles? 

[mlamouri (slow - plz ping), 2015/10/23 13:20:10]

No.
I did try on gitiles in case of it was a gitiles oddities. I'm glad it's not :)

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

But if you only want code markup, single backticks are still sufficient.

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

It would also be consistent with the code in the rest of the file, that uses
single backticks

+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

[Bernhard Bauer, 2015/10/22 16:19:12]

Nit: JUnit (capital U) here and elsewhere.

[mlamouri (slow - plz ping), 2015/10/23 13:20:10]

Done.

+
+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.
+
+There are two junit test suites: ```content_junit_tests``` and
+```chrome_junit_tests```.

[Bernhard Bauer, 2015/10/22 16:19:12]

There are more JUnit test suites than that, e.g. base_junit_tests,
components_junit_tests, net_junit_tests, ...

[dgn, 2015/10/22 18:09:23]

I wish there was a unified junit target with all the tests... It's not like it's
going to take as long as the instrumentation tests to build and run

[mlamouri (slow - plz ping), 2015/10/23 13:20:10]

Changed to mention content_junit_tests and chrome_junit_tests as examples.

+
+When adding a new junit test, the associated ```BUILD.gn``` file need to be
+updated. For ```chrome_junit_tests```, this is ```chrome/android/BUILD.gn``` and
+for ```content_junit_tests```, this is ```content/public/android/BUILD.gn```.
+
+```shell
+# Build the test suite.
+ninja -C out/Release content_junit_tests
+ninja -C out/Release chrome_junit_tests
+
+# Run the test suite.
+build/android/test_runner.py junit -s content_junit_tests --release -vvv
+build/android/test_runner.py junit -s chrome_junit_tests --release -vvv
+
+# Run a subset of tests. Contrary to gtests and instrumentation tests, the
+entire package name has to be used.

[dgn, 2015/10/22 18:09:23]

Not true. This is working for me:  net_junit_tests -f "*testGetNextAuthToken"   

[mlamouri (slow - plz ping), 2015/10/23 13:20:10]

Made the comment less assertive.

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

I think the full package name is matched against the filter, which would explain
the behavior Nicolas is seeing ☺

+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.