FindIt finds r575429 as the suspected culprit:

https://findit-for-me.appspot.com/waterfall/failure?redirect=1&url=https://ci.chromium.org/p/chromium/builders/luci.chromium.ci/Mac10.13%20Tests%20%28dbg%29/4076

Reverting...

The following revision refers to this bug:
  https://chromium.googlesource.com/chromium/src.git/+/7b8b4f583548218b816cff7bb4dbfef27ed81a56

commit 7b8b4f583548218b816cff7bb4dbfef27ed81a56
Author: Xiaocheng Hu <xiaochengh@chromium.org>
Date: Mon Jul 16 23:46:51 2018

Revert "Remove PlatformAccelerator."

This reverts commit 2f0772ee38f51f66d307338eb7fbd4d7d9a7e76b.

Reason for revert: Suspected to cause failure in AcceleratorsCocoaBrowserTest.MainMenuAcceleratorsInMapping

Original change's description:
> Remove PlatformAccelerator.
> 
> ui::Accelerator had a member platform_accelerator() which was only used on
> macOS, and provided redundant information. This CL removes the member.
> 
> Change-Id: Ib3954a2c8ff7197606c946e207dd5d3bd75af66d
> Bug:  846893 ,  702823 
> Reviewed-on: https://chromium-review.googlesource.com/1135644
> Commit-Queue: Erik Chen <erikchen@chromium.org>
> Reviewed-by: Nico Weber <thakis@chromium.org>
> Cr-Commit-Position: refs/heads/master@{#575429}

TBR=thakis@chromium.org,erikchen@chromium.org

Change-Id: I3f142628a1ab406028a5f64db7d6acdf13c4d41b
No-Presubmit: true
No-Tree-Checks: true
No-Try: true
Bug:  846893 ,  702823 ,  864272 
Reviewed-on: https://chromium-review.googlesource.com/1139153
Reviewed-by: Xiaocheng Hu <xiaochengh@chromium.org>
Commit-Queue: Xiaocheng Hu <xiaochengh@chromium.org>
Cr-Commit-Position: refs/heads/master@{#575484}
[modify] https://crrev.com/7b8b4f583548218b816cff7bb4dbfef27ed81a56/chrome/browser/global_keyboard_shortcuts_mac.mm
[modify] https://crrev.com/7b8b4f583548218b816cff7bb4dbfef27ed81a56/chrome/browser/ui/cocoa/accelerators_cocoa.h
[modify] https://crrev.com/7b8b4f583548218b816cff7bb4dbfef27ed81a56/chrome/browser/ui/cocoa/accelerators_cocoa.mm
[modify] https://crrev.com/7b8b4f583548218b816cff7bb4dbfef27ed81a56/chrome/browser/ui/cocoa/accelerators_cocoa_browsertest.mm
[add] https://crrev.com/7b8b4f583548218b816cff7bb4dbfef27ed81a56/chrome/browser/ui/cocoa/accelerators_cocoa_unittest.mm
[modify] https://crrev.com/7b8b4f583548218b816cff7bb4dbfef27ed81a56/chrome/browser/ui/cocoa/app_menu/app_menu_controller.h
[modify] https://crrev.com/7b8b4f583548218b816cff7bb4dbfef27ed81a56/chrome/browser/ui/cocoa/app_menu/app_menu_controller.mm
[modify] https://crrev.com/7b8b4f583548218b816cff7bb4dbfef27ed81a56/chrome/browser/ui/cocoa/app_menu/app_menu_controller_unittest.mm
[modify] https://crrev.com/7b8b4f583548218b816cff7bb4dbfef27ed81a56/chrome/browser/ui/cocoa/confirm_quit_panel_controller.h
[modify] https://crrev.com/7b8b4f583548218b816cff7bb4dbfef27ed81a56/chrome/browser/ui/cocoa/confirm_quit_panel_controller.mm
[modify] https://crrev.com/7b8b4f583548218b816cff7bb4dbfef27ed81a56/chrome/browser/ui/cocoa/confirm_quit_panel_controller_unittest.mm
[modify] https://crrev.com/7b8b4f583548218b816cff7bb4dbfef27ed81a56/chrome/browser/ui/cocoa/share_menu_controller.mm
[modify] https://crrev.com/7b8b4f583548218b816cff7bb4dbfef27ed81a56/chrome/browser/ui/cocoa/toolbar/reload_button_cocoa.mm
[modify] https://crrev.com/7b8b4f583548218b816cff7bb4dbfef27ed81a56/chrome/test/BUILD.gn
[modify] https://crrev.com/7b8b4f583548218b816cff7bb4dbfef27ed81a56/ui/base/BUILD.gn
[modify] https://crrev.com/7b8b4f583548218b816cff7bb4dbfef27ed81a56/ui/base/accelerators/accelerator.cc
[modify] https://crrev.com/7b8b4f583548218b816cff7bb4dbfef27ed81a56/ui/base/accelerators/accelerator.h
[add] https://crrev.com/7b8b4f583548218b816cff7bb4dbfef27ed81a56/ui/base/accelerators/platform_accelerator.h
[modify] https://crrev.com/7b8b4f583548218b816cff7bb4dbfef27ed81a56/ui/base/accelerators/platform_accelerator_cocoa.h
[modify] https://crrev.com/7b8b4f583548218b816cff7bb4dbfef27ed81a56/ui/base/accelerators/platform_accelerator_cocoa.mm
[modify] https://crrev.com/7b8b4f583548218b816cff7bb4dbfef27ed81a56/ui/base/cocoa/menu_controller.mm

Seems gone.

 Issue 864305  has been merged into this issue.