Add android_webview/docs/contributing-tests.md.

Test: Upload to gerrit > open file > click "gitiles"
Change-Id: I57a31e628e7aca2f73031603765b84ab88bcc4f9
Bug: 918221
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1894102
Commit-Queue: Andrew Luo <aluo@chromium.org>
Reviewed-by: Nate Fischer <ntfschr@chromium.org>
Cr-Commit-Position: refs/heads/master@{#712623}

android_webview/docs/contributing-tests.md

+# Contributing to WebView Tests
+
+[TOC]
+
+## Instrumentation tests
+These are on-device integration tests for android\_webview with rest of the
+chromium code (covering both Java and native code). A large percentage of
+Android applications use WebView, and could trigger WebView specific code paths
+in the codebase, therefore it is important to have solid integration tests.
+
+#### Where to add tests?
+The tests are located in the [javatests directory](/android_webview/javatests/src/org/chromium/android_webview/test/).
+They are roughly organized by the
+[Aw classes](/android_webview/java/src/org/chromium/android_webview/) (some
+tests are organized by overall functionality instead). Put new tests into an
+existing test class or create a new one if a suitable one isn't available.
+
+#### How to write intrumentation tests?
+See the [instrumentation test doc](/testing/android/docs/instrumentation.md).
+
+#### How do tests interact with WebView?
+Tests are written as JUnit4 tests. [AwActivityTestRule] is used to create and
+obtain references to WebView objects such as [AwContents](internally it launches
+the [WebView instrumentation shell](/android_webview/test/shell/src/org/chromium/android_webview/shell/)
+application to hold [AwTestContainerViews](/android_webview/test/shell/src/org/chromium/android_webview/test/AwTestContainerView.java)
+which in turn contains [AwContents]). [AwContents] will allow the test to
+trigger code paths such as loading urls, going forwards/backwards, etc... .
+[AwActivityTestRule] has some helper methods to call the [AwContents] methods,
+for example to ensure that they are called on the UI thread. Some AW
+components, such as [AwCookieManager](/android_webview/java/src/org/chromium/android_webview/AwCookieManager.java),
+can be directly created in tests.
+
+#### How do tests inject html/css/js content?
+Tests can use the load\* methods in [AwActivityTestRule] to inject snippets
+of content. This will however bypass the network layer. To have end-to-end
+testing, use [EmbeddedTestServer](/net/test/android/javatests/src/org/chromium/net/test/EmbeddedTestServer.java),
+which will allow simple loading of files from the [data directory](/android_webview/test/data/).
+For loading data from arbitrary URIs and more advanced control, the
+[TestWebServer](/net/test/android/javatests/src/org/chromium/net/test/util/TestWebServer.java)
+can be used.
+
+## Java unittest (JUnit)
+These are off-device tests using robolectric that only exercise android\_webview
+Java code.
+
+#### Where do add tests?
+The tests are located in the [junit directory](/android_webview/junit/src/org/chromium/android_webview/robolectric/).
+
+#### How to write junit tests?
+See the [JUnit doc](/testing/android/docs/junit.md#junit-tests-with-robolectric).
+
+## Native unittests
+These are on-device gtests that only exercise android\_webview native code.
+
+#### Where to add tests?
+The tests are located alongside respective source code files under
+[android\_webview directory](/android_webview/).
+
+#### How to write gtests?
+See the [GTest doc](/testing/android/docs/gtest_implementation.md).
+
+## How to run tests?
+Running tests is covered in [WebView Test Instructions](/android_webview/docs/test-instructions.md).
+
+[AwActivityTestRule]:
+/android_webview/javatests/src/org/chromium/android_webview/test/AwActivityTestRule.java
+[AwContents]:
+/android_webview/java/src/org/chromium/android_webview/AwContents.java