Assigning to avallee@chromium.org for triage and assignment. I'll disable this test for Mac in the meantime.

Looks like this might be related to https://chromium.googlesource.com/chromium/src/+/7da6ada25be4cdb742dddbfd6762bd90d872f4cf

The expected images for windows and linux were updated, but not for mac. Should it 

The following revision refers to this bug:
  https://chromium.googlesource.com/chromium/src.git/+/4abc8d7784a901b8ec43b8372635009e86fd4868

commit 4abc8d7784a901b8ec43b8372635009e86fd4868
Author: Scott Little <sclittle@chromium.org>
Date: Wed Nov 29 23:17:29 2017

Disable failing layout test editing/selection/4975120.html on Mac.

TBR=avallee@chromium.org

Bug:  789707 
Change-Id: Ie3beccb0ca4b22019f408a1edb485cc0108869d5
Reviewed-on: https://chromium-review.googlesource.com/798051
Reviewed-by: Scott Little <sclittle@chromium.org>
Commit-Queue: Scott Little <sclittle@chromium.org>
Cr-Commit-Position: refs/heads/master@{#520295}
[modify] https://crrev.com/4abc8d7784a901b8ec43b8372635009e86fd4868/third_party/WebKit/LayoutTests/TestExpectations

Removing Sheriff-Chromium because the test has been disabled.

The following revision refers to this bug:
  https://chromium.googlesource.com/chromium/src.git/+/be1e40e8555a6489a1bb76bd6c686df0f7fc2f23

commit be1e40e8555a6489a1bb76bd6c686df0f7fc2f23
Author: Alex Vallée <avallee@chromium.org>
Date: Thu Nov 30 14:56:39 2017

Add missing Mac test expectation from crrev.com/520215

Initial cl changed focus between iframes and the trybots didn't yell
about the missing mac update.

BUG= 755023 ,  789707 

Change-Id: I2bde8238088d3a9c43791c328cde423be810d939
Reviewed-on: https://chromium-review.googlesource.com/798755
Commit-Queue: Alex Vallee <avallee@chromium.org>
Reviewed-by: Philip Jägenstedt <foolip@chromium.org>
Cr-Commit-Position: refs/heads/master@{#520541}
[modify] https://crrev.com/be1e40e8555a6489a1bb76bd6c686df0f7fc2f23/third_party/WebKit/LayoutTests/TestExpectations
[modify] https://crrev.com/be1e40e8555a6489a1bb76bd6c686df0f7fc2f23/third_party/WebKit/LayoutTests/platform/mac/editing/selection/4975120-expected.png

webkit_layout_tests is now failing on the "Mac10.10 Tests" builder: https://ci.chromium.org/buildbot/chromium.mac/Mac10.10%20Tests/?limit=200

webkit_layout_tests on (none) GPU on Mac on Mac-10.10.5 webkit_layout_tests on (none) GPU on Mac on Mac-10.10.5
Run on OS: 'Mac-10.10.5'
Max pending time: 47s (shard #8)
Max shard duration: 0:09:21.460560 (shard #1)
Total tests: 78338
* Passed: 64749 (64586 expected, 163 unexpected)
* Skipped: 11806 (11806 expected, 0 unexpected)
* Failed: 1751 (1750 expected, >>>1 unexpected<<<)
* Flaky: 32 (32 expected, 0 unexpected)
 
Unexpected Failures:
* editing/selection/4975120.html

Please adjust expectations so that the 10.10 bot is happy in addition to all your other bots.

This is also failing on the https://uberchromegw.corp.google.com/i/chromium.webkit/builders/WebKit%20Mac10.10 bot. Given that that is the same bot mentioned in the original comment, please commit more baselines with care.

The following revision refers to this bug:
  https://chromium.googlesource.com/chromium/src.git/+/cf9bb47f3b68d4421cf7369d7aebaf51a41bf944

commit cf9bb47f3b68d4421cf7369d7aebaf51a41bf944
Author: Alex Vallée <avallee@chromium.org>
Date: Thu Nov 30 20:06:07 2017

Add missing update to mac-10.10 expectation.png.

TBR: rbyers, foolip
Bug:  789707 
Change-Id: I9ceb10e4ce74680a1d7d422a2d9391fd8b2a25c7
Reviewed-on: https://chromium-review.googlesource.com/801456
Reviewed-by: Alex Vallee <avallee@chromium.org>
Commit-Queue: Alex Vallee <avallee@chromium.org>
Cr-Commit-Position: refs/heads/master@{#520655}
[modify] https://crrev.com/cf9bb47f3b68d4421cf7369d7aebaf51a41bf944/third_party/WebKit/LayoutTests/platform/mac-mac10.10/editing/selection/4975120-expected.png

Build looks green now.