AW docs: improve public CTS docs

No change to logic, only docs. This expands on the existing CTS docs:

* Links to the AOSP source code
* Failures must be ReleaseBlock-Dev
* How to change test logic
* How to run tests locally

Fixed: 1138051
Test: Upload to gerrit > open file > click "gitiles"
Change-Id: I635718e5d1e4327a1964e617279282a157f14d72
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2542679
Auto-Submit: Nate Fischer <ntfschr@chromium.org>
Commit-Queue: Richard Coles <torne@chromium.org>
Reviewed-by: Richard Coles <torne@chromium.org>
Cr-Commit-Position: refs/heads/master@{#828288}

android_webview/tools/cts_config/README.md

@@ -57,22 +57,62 @@ please adjust them accordingly.
 
 ## Disabling/Skipping tests
 
-Add entries to the `excludes` list for the respective apk under `test_runs`.
+**CTS regressions are more serious than most test failures.** CTS failures block
+Android vendors from shipping devices and prevent the WebView team from dropping
+new Chrome and WebView APKs in the Android source tree. If you need to disable a
+test, please file a P1 crbug with **ReleaseBlock-Dev** in the `Mobile>WebView`
+component.
+
+If you must disable a test, you can add an entry to the `excludes` list for the
+correct apk (most tests belong to `CtsWebkitTestCases.apk`) under `test_runs`
+for each OS level which is failing.
 
 ## Re-enabling skipped tests
 
 Before re-enabling tests, make sure it's actually safe to enable the test again.
 
+* The test source code lives in Android and line numbers vary between OS
+  versions. You can find test code for a particular CTS release by finding the
+  appropriate git branch in codesearch:
+    * Lollipop: [lollipop-mr1-cts-release]
+    * Marshmallow: [marshmallow-cts-release]
+    * Nougat: [nougat-cts-dev]
+    * Oreo: [oreo-cts-dev]
+    * Pie: [pie-cts-dev]
+    * Android 10 (Q): [android10-tests-dev]
 * If the test was fixed on the Android side, the fix must be cherry-picked back
-  to the earliest applicable version. Ex. if the test was added in Android Oreo
-  (API 26), the fix should be picked back to `aosp/oreo-cts-dev`.
-    * **Note:** some OS levels are no longer supported by the CTS team, and will
+  to the earliest applicable version (see the git branches above). Ex. if the
+  test was added in Android Oreo (API 26), the fix should be picked back to
+  `aosp/oreo-cts-dev`.
+    * **Note:** some OS levels are no longer supported by the CTS team and will
       no longer receive CTS releases. Unfortunately, if there was a test bug for
       these OS levels, we must disable the test forever on that OS (and you
       should cherry-pick the fix to the earliest supported CTS branch).
 * If the failure was due to a chromium-side bug/regression, you can re-enable
-  the test as soon as the bug is fixed on trunk.
+  the test as soon as the bug is fixed on trunk. You can run CTS on a device or
+  emulator with [this guide](/android_webview/docs/test-instructions.md#cts).
 
 Re-enabling the test case is as simple as removing it from the `excludes` for
 the relevant OS levels. Please verify this change by adding the
 `android-webview-*` trybots (not enabled by default).
+
+## Changing CTS tests retroactively
+
+Android generally has strict backward compatibility requirements, and this
+extends to CTS. However, sometimes it's appropriate to change the test logic
+rather than restoring the old chromium behavior, such as when the test logic is
+responsible for flakiness or relies on an invalid assumption. Please reach out
+to [the WebView team][1] quickly if you think a CTS test needs to change (the
+failure is still considered **ReleaseBlock-Dev** until the test change actually
+lands in Android).
+
+Any CTS changes must be backward compatible. The original WebView version which
+shipped on that OS version must continue to pass the revised CTS test.
+
+[1]: https://groups.google.com/a/chromium.org/forum/#!forum/android-webview-dev
+[lollipop-mr1-cts-release]: https://cs.android.com/android/platform/superproject/+/lollipop-mr1-cts-release:cts/tests/tests/webkit/src/android/webkit/cts/
+[marshmallow-cts-release]: https://cs.android.com/android/platform/superproject/+/marshmallow-cts-release:cts/tests/tests/webkit/src/android/webkit/cts/
+[nougat-cts-dev]: https://cs.android.com/android/platform/superproject/+/nougat-cts-dev:cts/tests/tests/webkit/src/android/webkit/cts/
+[oreo-cts-dev]: https://cs.android.com/android/platform/superproject/+/oreo-cts-dev:cts/tests/tests/webkit/src/android/webkit/cts/
+[pie-cts-dev]: https://cs.android.com/android/platform/superproject/+/pie-cts-dev:cts/tests/tests/webkit/src/android/webkit/cts/
+[android10-tests-dev]: https://cs.android.com/android/platform/superproject/+/android10-tests-dev:cts/tests/tests/webkit/src/android/webkit/cts/