Yep, that CL is the cause, and changing that baseline was unintended.

Will follow up by reverting that change to that baseline.

The following revision refers to this bug:
  https://chromium.googlesource.com/chromium/src.git/+/ffd1654da7e07244dab34c86cceafc8a2e6e18ec

commit ffd1654da7e07244dab34c86cceafc8a2e6e18ec
Author: qyearsley <qyearsley@chromium.org>
Date: Tue Feb 21 00:52:36 2017

Revert change to ".../calendar-picker-appearance-zoom12..." mac 10.9 baseline.

TBR=tdanderson
TBR_REASON=Making the waterfall green again, undoing minor accidental change
BUG= 694338 

Review-Url: https://codereview.chromium.org/2707993002
Cr-Commit-Position: refs/heads/master@{#451678}

[modify] https://crrev.com/ffd1654da7e07244dab34c86cceafc8a2e6e18ec/third_party/WebKit/LayoutTests/platform/mac-mac10.9/fast/forms/calendar-picker/calendar-picker-appearance-zoom125-expected.png

The tests still failures: https://build.chromium.org/p/chromium.webkit/builders/WebKit%20Mac10.9/builds/43063

Mark those tests as failures https://codereview.chromium.org/2723443002

The following revision refers to this bug:
  https://chromium.googlesource.com/chromium/src.git/+/642a66998d9cc9c4151a9c3284e43f805de7b590

commit 642a66998d9cc9c4151a9c3284e43f805de7b590
Author: hajimehoshi <hajimehoshi@chromium.org>
Date: Mon Feb 27 08:37:49 2017

Mark calendar-picker-appearance-zoom125/200.html as failures

BUG= 694338 
TBR=hajimehoshi@chromium.org
NOTRY=true

Review-Url: https://codereview.chromium.org/2723443002
Cr-Commit-Position: refs/heads/master@{#453172}

[modify] https://crrev.com/642a66998d9cc9c4151a9c3284e43f805de7b590/third_party/WebKit/LayoutTests/TestExpectations

This seems to be fixed, so I'm removing sheriff label.

Summary of this bug:

 1. This test started failing when baselines were accidentally changed in a wpt import CL, because the test failed on try bots
 2. Then previous baselines were added again, but the test was still failing -- maybe the failures on try bots before werne't one-off flaky failures?
 3. Now trying to rebaseline again: https://codereview.chromium.org/2722593002

The following revision refers to this bug:
  https://chromium.googlesource.com/chromium/src.git/+/38c8282555876f13213276ff077bf449a1c70a4d

commit 38c8282555876f13213276ff077bf449a1c70a4d
Author: qyearsley <qyearsley@chromium.org>
Date: Tue Feb 28 17:33:38 2017

Rebaseline calendar-picker-appearance-zoom125/200.html.

This CL would download the latest actual results for Mac10.9 from the
continuous builder for calendar-picker-appearance-zoom125/200.html.

BUG= 694338 

Review-Url: https://codereview.chromium.org/2722593002
Cr-Commit-Position: refs/heads/master@{#453631}

[modify] https://crrev.com/38c8282555876f13213276ff077bf449a1c70a4d/third_party/WebKit/LayoutTests/TestExpectations
[modify] https://crrev.com/38c8282555876f13213276ff077bf449a1c70a4d/third_party/WebKit/LayoutTests/platform/mac-mac10.9/fast/forms/calendar-picker/calendar-picker-appearance-zoom125-expected.png
[modify] https://crrev.com/38c8282555876f13213276ff077bf449a1c70a4d/third_party/WebKit/LayoutTests/platform/mac-mac10.9/fast/forms/calendar-picker/calendar-picker-appearance-zoom200-expected.png

The following revision refers to this bug:
  https://chromium.googlesource.com/chromium/src.git/+/80455f9645fbb7d576e4ab83c1d78acf51cd4792

commit 80455f9645fbb7d576e4ab83c1d78acf51cd4792
Author: sergeyu <sergeyu@chromium.org>
Date: Wed Mar 01 00:48:40 2017

Revert of Rebaseline calendar-picker-appearance-zoom125/200.html. (patchset #2 id:20001 of https://codereview.chromium.org/2722593002/ )

Reason for revert:
The test still fails: https://uberchromegw.corp.google.com/i/chromium.webkit/builders/WebKit%20Mac10.9/builds/43188 . Reverting this CL.

Original issue's description:
> Rebaseline calendar-picker-appearance-zoom125/200.html.
>
> This CL would download the latest actual results for Mac10.9 from the
> continuous builder for calendar-picker-appearance-zoom125/200.html.
>
> BUG= 694338 
>
> Review-Url: https://codereview.chromium.org/2722593002
> Cr-Commit-Position: refs/heads/master@{#453631}
> Committed: https://chromium.googlesource.com/chromium/src/+/38c8282555876f13213276ff077bf449a1c70a4d

TBR=hajimehoshi@chromium.org,qyearsley@chromium.org
# Skipping CQ checks because original CL landed less than 1 days ago.
NOPRESUBMIT=true
NOTREECHECKS=true
NOTRY=true
BUG= 694338 

Review-Url: https://codereview.chromium.org/2722963002
Cr-Commit-Position: refs/heads/master@{#453779}

[modify] https://crrev.com/80455f9645fbb7d576e4ab83c1d78acf51cd4792/third_party/WebKit/LayoutTests/TestExpectations
[modify] https://crrev.com/80455f9645fbb7d576e4ab83c1d78acf51cd4792/third_party/WebKit/LayoutTests/platform/mac-mac10.9/fast/forms/calendar-picker/calendar-picker-appearance-zoom125-expected.png
[modify] https://crrev.com/80455f9645fbb7d576e4ab83c1d78acf51cd4792/third_party/WebKit/LayoutTests/platform/mac-mac10.9/fast/forms/calendar-picker/calendar-picker-appearance-zoom200-expected.png

It's possible that these tests are actually flaky, but it's probably worth trying once more to rebaseline (and check whether the new baselines are OK) for these two tests:

fast/forms/calendar-picker/calendar-picker-appearance-zoom125.html
fast/forms/calendar-picker/calendar-picker-appearance-zoom200.html

This issue has been Available for over a year. If it's no longer important or seems unlikely to be fixed, please consider closing it out. If it is important, please re-triage the issue.

Sorry for the inconvenience if the bug really should have been left as Available.

For more details visit https://www.chromium.org/issue-tracking/autotriage - Your friendly Sheriffbot

This only seems to be an issue with slimming paint v2. Reopen if this is not the case.