From a bisect, and confirmed by a local revert, this seems to be caused by:

commit eccac1fd6db85731fb95b11915fc2197a6ceb0c3
Author: pdr <pdr@chromium.org>
Date:   Mon Sep 12 22:13:49 2016 -0700

    Do not let text-size-adjust override accessibility font settings
    
    The text autosizer is used for both making desktop pages more legible
    on mobile, and for applying the accessibility font scale factor. When
    text-size-adjust support was added, pages were able to override the
    accessibility setting in addition to the autosizing multiplier.
    
    This patch ensures accessibility font scale settings are respected even
    with text-size-adjust. A followup bug has been filed (crbug.com/645717)
    for moving the accessibility font scale factor out of the autosizer.
    
    This patch also re-enables support for text-size-adjust which was
    disabled temporarily due to breaking accessibility settings.
    
    BUG= 645269 
    
    Review-Url: https://codereview.chromium.org/2329733002
    Cr-Commit-Position: refs/heads/master@{#418173}

I will revert this on master, although it is not obvious to me what the relationship is.

The following revision refers to this bug:
  https://chromium.googlesource.com/chromium/src.git/+/3ec856cdfb31f023a832547f4f702858e9b8a903

commit 3ec856cdfb31f023a832547f4f702858e9b8a903
Author: aberent <aberent@chromium.org>
Date: Tue Sep 13 17:05:51 2016

Revert of Do not let text-size-adjust override accessibility font settings (patchset #2 id:20001 of https://codereview.chromium.org/2329733002/ )

Reason for revert:
Broke ContextualSearchManagerTest#testChainedSearchCreatesNewContent and ContextualSearchManagerTest#testChainedTapsRemovedFromHistory

BUG= 646342 

Original issue's description:
> Do not let text-size-adjust override accessibility font settings
>
> The text autosizer is used for both making desktop pages more legible
> on mobile, and for applying the accessibility font scale factor. When
> text-size-adjust support was added, pages were able to override the
> accessibility setting in addition to the autosizing multiplier.
>
> This patch ensures accessibility font scale settings are respected even
> with text-size-adjust. A followup bug has been filed (crbug.com/645717)
> for moving the accessibility font scale factor out of the autosizer.
>
> This patch also re-enables support for text-size-adjust which was
> disabled temporarily due to breaking accessibility settings.
>
> BUG= 645269 
>
> Committed: https://crrev.com/eccac1fd6db85731fb95b11915fc2197a6ceb0c3
> Cr-Commit-Position: refs/heads/master@{#418173}

TBR=skobes@chromium.org,pdr@chromium.org
# Skipping CQ checks because original CL landed less than 1 days ago.
NOPRESUBMIT=true
NOTREECHECKS=true
NOTRY=true
BUG= 645269 

Review-Url: https://codereview.chromium.org/2335193003
Cr-Commit-Position: refs/heads/master@{#418278}

[modify] https://crrev.com/3ec856cdfb31f023a832547f4f702858e9b8a903/third_party/WebKit/Source/core/layout/TextAutosizer.cpp
[modify] https://crrev.com/3ec856cdfb31f023a832547f4f702858e9b8a903/third_party/WebKit/Source/core/layout/TextAutosizer.h
[modify] https://crrev.com/3ec856cdfb31f023a832547f4f702858e9b8a903/third_party/WebKit/Source/core/layout/TextAutosizerTest.cpp

Assigning to pdr@ as the owner of the reverted CL.
+pedrosimonetti@ as the author of the tests (I believe).

Thanks for the rollout. This was in fact a typo in my patch.

I'm relanding in https://codereview.chromium.org/2340553003 with a test that has coverage on all bots. There are no trybots for the configurations that initially failed so I'll be watching the builders as this lands.

The following revision refers to this bug:
  https://chromium.googlesource.com/chromium/src.git/+/1229b1414d64c314666d2fb3bf47706f2b5e947b

commit 1229b1414d64c314666d2fb3bf47706f2b5e947b
Author: pdr <pdr@chromium.org>
Date: Wed Sep 14 04:40:53 2016

(reland) Do not let text-size-adjust override accessibility font settings

The text autosizer is used for both making desktop pages more legible
on mobile, and for applying the accessibility font scale factor. When
text-size-adjust support was added, pages were able to override the
accessibility setting in addition to the autosizing multiplier.

This patch ensures accessibility font scale settings are respected even
with text-size-adjust. A followup bug has been filed (crbug.com/645717)
for moving the accessibility font scale factor out of the autosizer.

This patch also re-enables support for text-size-adjust which was
disabled temporarily due to breaking accessibility settings.

This was reverted due to a forgotten "!" in checking whether the
viewport was specified by the author in TextAutosizer.cpp. A new test
has been added to catch this in the future. The unneeded meta viewport
declarations in all other tests have been removed because they had
no effect with the meta viewport setting disabled.

Original review:
https://codereview.chromium.org/2329733002

BUG= 645269 , 646342 

Review-Url: https://codereview.chromium.org/2340553003
Cr-Commit-Position: refs/heads/master@{#418488}

[modify] https://crrev.com/1229b1414d64c314666d2fb3bf47706f2b5e947b/third_party/WebKit/Source/core/layout/TextAutosizer.cpp
[modify] https://crrev.com/1229b1414d64c314666d2fb3bf47706f2b5e947b/third_party/WebKit/Source/core/layout/TextAutosizer.h
[modify] https://crrev.com/1229b1414d64c314666d2fb3bf47706f2b5e947b/third_party/WebKit/Source/core/layout/TextAutosizerTest.cpp

The bots now look happy, the engineers look as happy as engineers can, closing.

The following revision refers to this bug:
  https://chromium.googlesource.com/chromium/src.git/+/d25ac697753f02ac5c923d65402d75b4211cab28

commit d25ac697753f02ac5c923d65402d75b4211cab28
Author: pdr@chromium.org <pdr@chromium.org>
Date: Tue Sep 20 17:59:25 2016

(reland) Do not let text-size-adjust override accessibility font settings

The text autosizer is used for both making desktop pages more legible
on mobile, and for applying the accessibility font scale factor. When
text-size-adjust support was added, pages were able to override the
accessibility setting in addition to the autosizing multiplier.

This patch ensures accessibility font scale settings are respected even
with text-size-adjust. A followup bug has been filed (crbug.com/645717)
for moving the accessibility font scale factor out of the autosizer.

This patch also re-enables support for text-size-adjust which was
disabled temporarily due to breaking accessibility settings.

This was reverted due to a forgotten "!" in checking whether the
viewport was specified by the author in TextAutosizer.cpp. A new test
has been added to catch this in the future. The unneeded meta viewport
declarations in all other tests have been removed because they had
no effect with the meta viewport setting disabled.

Original review:
https://codereview.chromium.org/2329733002

BUG= 645269 , 646342 

Review-Url: https://codereview.chromium.org/2340553003
Cr-Commit-Position: refs/heads/master@{#418488}
(cherry picked from commit 1229b1414d64c314666d2fb3bf47706f2b5e947b)

Review URL: https://codereview.chromium.org/2355053002 .

Cr-Commit-Position: refs/branch-heads/2840@{#444}
Cr-Branched-From: 1ae106dbab4bddd85132d5b75c670794311f4c57-refs/heads/master@{#414607}

[modify] https://crrev.com/d25ac697753f02ac5c923d65402d75b4211cab28/third_party/WebKit/Source/core/layout/TextAutosizer.cpp
[modify] https://crrev.com/d25ac697753f02ac5c923d65402d75b4211cab28/third_party/WebKit/Source/core/layout/TextAutosizer.h
[modify] https://crrev.com/d25ac697753f02ac5c923d65402d75b4211cab28/third_party/WebKit/Source/core/layout/TextAutosizerTest.cpp

The following revision refers to this bug:
  https://chromium.googlesource.com/chromium/src.git/+/d25ac697753f02ac5c923d65402d75b4211cab28

commit d25ac697753f02ac5c923d65402d75b4211cab28
Author: pdr@chromium.org <pdr@chromium.org>
Date: Tue Sep 20 17:59:25 2016

(reland) Do not let text-size-adjust override accessibility font settings

The text autosizer is used for both making desktop pages more legible
on mobile, and for applying the accessibility font scale factor. When
text-size-adjust support was added, pages were able to override the
accessibility setting in addition to the autosizing multiplier.

This patch ensures accessibility font scale settings are respected even
with text-size-adjust. A followup bug has been filed (crbug.com/645717)
for moving the accessibility font scale factor out of the autosizer.

This patch also re-enables support for text-size-adjust which was
disabled temporarily due to breaking accessibility settings.

This was reverted due to a forgotten "!" in checking whether the
viewport was specified by the author in TextAutosizer.cpp. A new test
has been added to catch this in the future. The unneeded meta viewport
declarations in all other tests have been removed because they had
no effect with the meta viewport setting disabled.

Original review:
https://codereview.chromium.org/2329733002

BUG= 645269 , 646342 

Review-Url: https://codereview.chromium.org/2340553003
Cr-Commit-Position: refs/heads/master@{#418488}
(cherry picked from commit 1229b1414d64c314666d2fb3bf47706f2b5e947b)

Review URL: https://codereview.chromium.org/2355053002 .

Cr-Commit-Position: refs/branch-heads/2840@{#444}
Cr-Branched-From: 1ae106dbab4bddd85132d5b75c670794311f4c57-refs/heads/master@{#414607}

[modify] https://crrev.com/d25ac697753f02ac5c923d65402d75b4211cab28/third_party/WebKit/Source/core/layout/TextAutosizer.cpp
[modify] https://crrev.com/d25ac697753f02ac5c923d65402d75b4211cab28/third_party/WebKit/Source/core/layout/TextAutosizer.h
[modify] https://crrev.com/d25ac697753f02ac5c923d65402d75b4211cab28/third_party/WebKit/Source/core/layout/TextAutosizerTest.cpp