[Android] Add gtest implementation documentation.

testing/android/docs/gtest_implementation.md

+# How GTests work on Android
+
+gtests are [googletest](https://github.com/google/googletest)-based C++ tests.
+On Android, they run on a device. In most cases, they're packaged as APKs, but
+there are a few cases where they're run as raw executables. The latter is
+necessary in a few cases, particularly when manipulating signal handlers, but
+isn't possible when the suite needs to call back through the JNI into Java code.
+
+[TOC]
+
+## APKs
+
+### GN
+
+Gtest APKs are built by default by the
+[test](https://codesearch.chromium.org/chromium/src/testing/test.gni?type=cs&q=file:%5Esrc%5C/testing%5C/test.gni$+template%5C("test"%5C)&sq=package:chromium)
+template, e.g.
+
+```python
+test("sample_gtest") {
+  # ...
+}
+```
+
+This uses gn's native
+[shared_library](https://chromium.googlesource.com/chromium/src/+/master/tools/gn/docs/reference.md#shared_library_Declare-a-shared-library-target)
+target type along with the
+[unittest_apk](https://codesearch.chromium.org/chromium/src/build/config/android/rules.gni?type=cs&q=file:%5Esrc%5C/build%5C/config%5C/android%5C/rules.gni$+template%5C(%5C"unittest_apk%5C"%5C)&sq=package:chromium)
+template to build an APK containing:
+
+ - One or more .so files containing the native code on which the test suite
+depends
+ - One or more .dex files containing the Java code on which the test suite
+depends
+ - A [manifest](https://developer.android.com/guide/topics/manifest/manifest-intro.html)
+file that contains `<instrumentation>` and `<activity>` elements (among others).
+
+### Harness
+
+GTest APKs are packaged with a harness that consists of:
+
+  - [NativeTestInstrumentationTestRunner], an instrumentation entry point that
+handles running one or more sequential instances of a test Activity. Typically,
+unit test suites will only use one instance of the Activity and will run all of
+the specified tests in it, while browser test suites will use multiple instances
+and will only run one test per instance.
+  - Three [Activity](https://developer.android.com/reference/android/app/Activity.html)-based
+classes
+([NativeUnitTestActivity](https://codesearch.chromium.org/chromium/src/testing/android/native_test/java/src/org/chromium/native_test/NativeUnitTestActivity.java),
+[NativeUnitTestNativeActivity](https://codesearch.chromium.org/chromium/src/testing/android/native_test/java/src/org/chromium/native_test/NativeUnitTestNativeActivity.java),
+and
+[NativeBrowserTestActivity](https://codesearch.chromium.org/chromium/src/testing/android/native_test/java/src/org/chromium/native_test/NativeBrowserTestActivity.java))
+that primarily act as process entry points for individual test shards.
+Only one is used in any given suite.
+  - [NativeTest] and [NativeUnitTest],
+which handle formatting arguments for googletest and transferring control across
+the JNI.
+  - [testing::android::RunTests](https://codesearch.chromium.org/chromium/src/testing/android/native_test/native_test_launcher.cc?q=file:%5Esrc%5C/testing%5C/android%5C/native_test%5C/native_test_launcher.cc$+RunTests&sq=package:chromium),
+the function on the native side, which initializes the native command-line,
+redirects stdout either to a FIFO or a regular file, optionally waits for a
+debugger to attach to the process, sets up the test data directories, and then
+dispatches to googletest's `main` function.
+
+### Runtime
+
+ 1. The test runner calls `am instrument` with a bunch of arguments,
+    includes several extras that are arguments to either
+    [NativeTestInstrumentationTestRunner] or [NativeTest]. This results in an
+    intent being sent to [NativeTestInstrumentationTestRunner].
+ 2. [NativeTestInstrumentationTestRunner] is created. In its onCreate, it
+    parses its own arguments from the intent and retains all other arguments
+    to be passed to the Activities it'll start later. It also creates a
+    temporary file in the external storage directory for stdout. It finally
+    starts itself.
+ 3. [NativeTestInstrumentationTestRunner] is started. In its onStart, it prepares
+    to receive notifications about the start and end of the test run from the
+    Activities it's about to start. It then creates [ShardStarter]
+    that will start the first test shard and adds that to the current
+    [Handler](https://developer.android.com/reference/android/os/Handler.html).
+ 4. The [ShardStarter] is executed, starting the test Activity.
+ 5. The Activity starts, possibly doing some process initialization, and hands
+    off to the [NativeTest].
+ 6. The [NativeTest] handles some initialization and informs the
+    [NativeTestInstrumentationTestRunner] that it has started. On hearing this,
+    the [NativeTestInstrumentationTestRunner] creates a [ShardMonitor] 
+    that will monitor the execution of the test Activity.
+ 7. The [NativeTest] hands off to testing::android::RunTests. The tests run.
+ 8. The [NativeTest] informs the [NativeTestInstrumentationTestRunner] that is has
+    completed. On hearing this, the [ShardMonitor] creates a [ShardEnder].
+ 9. The [ShardEnder] is executed, killing the child process (if applicable),
+    parsing the results from the stdout file, and either launching the next
+    shard via [ShardStarter] (in which case the process returns to #4) or sending
+    the results out to the test runner and finishing the instrumentation.
+
+## Executables
+
+### GN
+
+Gtest executables are built by passing
+`use_raw_android_executable = True` to the 
+[test](https://codesearch.chromium.org/chromium/src/testing/test.gni?type=cs&q=file:%5Esrc%5C/testing%5C/test.gni$+template%5C("test"%5C)&sq=package:chromium)
+template, e.g.
+
+```python
+test("sample_gtest_executable") {
+  if (is_android) {
+    use_raw_android_executable = true
+  }
+  # ...
+}
+```
+
+This uses gn's native
+[executable](https://chromium.googlesource.com/chromium/src/+/master/tools/gn/docs/reference.md#executable_Declare-an-executable-target)
+target type, then copies the resulting executable and any requisite shared libraries
+to ```${root_out_dir}/${target_name}__dist``` (e.g. ```out/Debug/breakpad_unittests__dist```).
+
+### Harness
+
+Unlike APKs, gtest suites built as executables require no Android-specific harnesses.
+
+### Runtime
+
+The test runner simply executes the binary on the device directly and parses the
+stdout on its own.
+
+[NativeTest]: https://codesearch.chromium.org/chromium/src/testing/android/native_test/java/src/org/chromium/native_test/NativeTest.java
+[NativeTestInstrumentationTestRunner]: https://codesearch.chromium.org/chromium/src/testing/android/native_test/java/src/org/chromium/native_test/NativeTestInstrumentationTestRunner.java
+[NativeUnitTest]: https://codesearch.chromium.org/chromium/src/testing/android/native_test/java/src/org/chromium/native_test/NativeUnitTest.java
+[ShardEnder]: https://codesearch.chromium.org/chromium/src/testing/android/native_test/java/src/org/chromium/native_test/NativeTestInstrumentationTestRunner.java?q=file:NativeTestInstrumentationTestRunner.java+class:ShardEnder&sq=package:chromium
+[ShardMonitor]: https://codesearch.chromium.org/chromium/src/testing/android/native_test/java/src/org/chromium/native_test/NativeTestInstrumentationTestRunner.java?q=file:NativeTestInstrumentationTestRunner.java+class:ShardMonitor&sq=package:chromium
+[ShardStarter]: https://codesearch.chromium.org/chromium/src/testing/android/native_test/java/src/org/chromium/native_test/NativeTestInstrumentationTestRunner.java?q=file:NativeTestInstrumentationTestRunner.java+class:ShardStarter&sq=package:chromium