I don't remember this ever being marked flaky . There was a huge refactor to Mojo recently. I wonder if some change broke it after. 

question is: how can we easily find which CL made it flaky?

https://build.chromium.org/p/chromium.linux/builders/Android%20Tests%20%28dbg%29/builds/41484 was the most recent failure.

Finding the CL that introduced flakiness is hard, but I usually start by looking for the earliest (recent) failure:
https://test-results.appspot.com/dashboards/flakiness_dashboard.html#testType=android_webview_test_apk&showAllRuns=true&tests=org.chromium.android_webview.test.PostMessageTest%23testCanUseReceivedAwMessagePortFromJS

These are the oldest failures I can find on each line:
https://build.chromium.org/p/chromium.linux/builders/Android%20Tests/builds/40214
https://build.chromium.org/p/chromium.linux/builders/Android%20Tests/builds/39944
https://build.chromium.org/p/chromium.linux/builders/Android%20Tests%20(dbg)/builds/41484
https://build.chromium.org/p/chromium.swarm/builders/Android%20N5%20Swarm/builds/126
https://build.chromium.org/p/chromium.android/builders/Marshmallow%20Tablet%20Tester/builds/8011

The oldest of them was https://build.chromium.org/p/chromium.swarm/builders/Android%20N5%20Swarm/builds/126 built from https://chromium.googlesource.com/chromium/src/+/9d7a8aef95959ab06d2b565fe129fd27ed42b963 so if the flakiness was introduced by a code change, it might be not too long before that.

Both the oldest and the newest failure is like this:

org.chromium.android_webview.test.PostMessageTest#testCanUseReceivedAwMessagePortFromJS with {--webview-sandboxed-renderer} (run #1):
junit.framework.AssertionFailedError: Values did not match. Expected: 24, actual: 2
	at org.chromium.content.browser.test.util.CriteriaHelper.pollInstrumentationThread(CriteriaHelper.java:79)
	at org.chromium.content.browser.test.util.CriteriaHelper.pollUiThread(CriteriaHelper.java:117)
	at org.chromium.content.browser.test.util.CriteriaHelper.pollUiThread(CriteriaHelper.java:138)
	at org.chromium.android_webview.test.PostMessageTest.expectTitle(PostMessageTest.java:160)
	at org.chromium.android_webview.test.PostMessageTest.testCanUseReceivedAwMessagePortFromJS(PostMessageTest.java:798)
	at java.lang.reflect.Method.invokeNative(Native Method)
	at android.test.InstrumentationTestCase.runMethod(InstrumentationTestCase.java:214)
	at android.test.InstrumentationTestCase.runTest(InstrumentationTestCase.java:199)
	at android.test.ActivityInstrumentationTestCase2.runTest(ActivityInstrumentationTestCase2.java:192)
	at org.chromium.base.test.BaseTestResult.runParameterized(BaseTestResult.java:161)
	at org.chromium.base.test.BaseTestResult.run(BaseTestResult.java:124)
	at android.test.AndroidTestRunner.runTest(AndroidTestRunner.java:191)
	at android.test.AndroidTestRunner.runTest(AndroidTestRunner.java:176)
	at android.test.InstrumentationTestRunner.onStart(InstrumentationTestRunner.java:554)
	at android.app.Instrumentation$InstrumentationThread.run(Instrumentation.java:1701)

A CL to disable the test is going through CQ in https://codereview.chromium.org/2801983002 so I'm removing the Sheriff-Chromium label. If it's impossible to make any progress on this issue with the test disabled, reverting that would be fine, failures are rare enough that it should be manageable for a day or two.

The following revision refers to this bug:
  https://chromium.googlesource.com/chromium/src.git/+/0cfc2fe30fabcd67f5b8b5fe158393ca194d8b45

commit 0cfc2fe30fabcd67f5b8b5fe158393ca194d8b45
Author: foolip <foolip@chromium.org>
Date: Thu Apr 06 11:44:57 2017

Disable flaky testCanUseReceivedAwMessagePortFromJS() test

BUG= 708821 
TBR=sgurun@chromium.org

Review-Url: https://codereview.chromium.org/2801983002
Cr-Commit-Position: refs/heads/master@{#462430}

[modify] https://crrev.com/0cfc2fe30fabcd67f5b8b5fe158393ca194d8b45/android_webview/javatests/src/org/chromium/android_webview/test/PostMessageTest.java

bulk editing to assign, rather than available. 

looped test locally 50 times and no failures. gonna try enabling with RetryOnFailure. I guess someone check back in a couple of weeks and see if there are any flakes on the flakiness dashboard

https://chromium-review.googlesource.com/c/chromium/src/+/685307

fwiw, webview tests no longer run on Android Tests, which is kitkat. it does still run on M though

The following revision refers to this bug:
  https://chromium.googlesource.com/chromium/src.git/+/5eb217143db8fa07fa115b3efef1ccd35304c6ef

commit 5eb217143db8fa07fa115b3efef1ccd35304c6ef
Author: Bo Liu <boliu@chromium.org>
Date: Tue Sep 26 23:24:28 2017

aw: Enable testCanUseReceivedAwMessagePortFromJS with RetryOnFailure

Can't reproduce flake locally by looping test.

Bug:  708821 
Change-Id: I9ad32ce87b1d821c7028c88b80539af903aed453
Reviewed-on: https://chromium-review.googlesource.com/685307
Reviewed-by: Paul Miller <paulmiller@chromium.org>
Commit-Queue: Bo <boliu@chromium.org>
Cr-Commit-Position: refs/heads/master@{#504518}
[modify] https://crrev.com/5eb217143db8fa07fa115b3efef1ccd35304c6ef/android_webview/javatests/src/org/chromium/android_webview/test/PostMessageTest.java

Looks pretty good. Not a single retry flake so far:
https://test-results.appspot.com/dashboards/flakiness_dashboard.html#testType=webview_instrumentation_test_apk&tests=org.chromium.android_webview.test.PostMessageTest%23testCanUseReceivedAwMessagePortFromJS

The following revision refers to this bug:
  https://chromium.googlesource.com/chromium/src.git/+/e6d662784e4615e8f268bc40c327330138762ea1

commit e6d662784e4615e8f268bc40c327330138762ea1
Author: Bo Liu <boliu@chromium.org>
Date: Sun Oct 01 03:53:31 2017

aw: Remove RetryOnFailure for testCanUseReceivedAwMessagePortFromJS

Looks good on cq for 3 days.

Bug:  708821 
Change-Id: Ia7faf371c8449a36d5912c5c9f4128e23bdf5170
Reviewed-on: https://chromium-review.googlesource.com/693406
Reviewed-by: Paul Miller <paulmiller@chromium.org>
Commit-Queue: Bo <boliu@chromium.org>
Cr-Commit-Position: refs/heads/master@{#505495}
[modify] https://crrev.com/e6d662784e4615e8f268bc40c327330138762ea1/android_webview/javatests/src/org/chromium/android_webview/test/PostMessageTest.java