So this failure only happens on Mac-10.11? How many bots are running on Mac-10.11 and how many of them have such a failure? And how often?

The related codes haven't been touched for more than half year since landing and I can't reproduce it on my MBP 10.12, as mentioned in  issue759900 .

I'll see if I can get a MBP10.11 loaner machine for a test.

This failed again on 10.11:

https://uberchromegw.corp.google.com/i/chromium.mac/builders/Mac10.11%20Tests/builds/18390

Bumping priority. braveyao@, can you take a peek?

Sorry for the delay due to my OOO.

Still didn't get the Mac10.11 machine. 
But most probably, this wouldn't be reproduced locally. Then I will add some debugging info and see if we can isolate the issue to OSX side or Chrome side. And take some action according to it.

Questions: is there any mouse or touch-pad connected to the mac bots and is there any possibility that someone may move the mouse cursor during tests running?

If the answers are YES, then I think that's the reason. In those test cases I need to actually move mouse cursor into/out_of certain range with OS API. If the cursor is moved by real device at same time, the the position will be unexpected and test case may fail. 

The following revision refers to this bug:
  https://chromium.googlesource.com/chromium/src.git/+/1e6094789708c75e7dd0628eec61df502154cbf2

commit 1e6094789708c75e7dd0628eec61df502154cbf2
Author: braveyao <braveyao@chromium.org>
Date: Wed Oct 11 21:12:35 2017

CursorRendererMacTest: dis-associate mouse and cursor in testing

The CursorRendererMacTest may fail under certain occasions on the bots.
So far the only possible replication I can find on local machine is to
move mouse during tests running. It seems that when both test app and
mouse both try to manipulate cursor at same time, we'll get unexpected
cursor position and tests will fail.

One way to solve this issue is to intercept and discard the
kCGEventMouseMoved event and only let the test control cursor position.
With this fix, moving mouse during test won't let test down locally.

Bug:  761902 
Change-Id: I6519110167b32292228bc5522908d8dbce1fff66
Reviewed-on: https://chromium-review.googlesource.com/710865
Reviewed-by: Yuri Wiitala <miu@chromium.org>
Commit-Queue: Weiyong Yao <braveyao@chromium.org>
Cr-Commit-Position: refs/heads/master@{#508115}
[modify] https://crrev.com/1e6094789708c75e7dd0628eec61df502154cbf2/content/browser/media/capture/cursor_renderer_mac_unittest.mm

Landed a cl to disconnect mouse and cursor during CursorRendererMacTest. Let see if it will fix the problem on bots.