https://chromium-review.googlesource.com/c/chromium/src/+/641402 is suspicious. Going to speculatively revert to see if it helps.

The following revision refers to this bug:
  https://chromium.googlesource.com/chromium/src.git/+/0029db1595c245cbe93dd8b6f3039e9af625a452

commit 0029db1595c245cbe93dd8b6f3039e9af625a452
Author: Tarun Bansal <tbansal@chromium.org>
Date: Wed Aug 30 00:16:55 2017

Speculatively revert "Mojo: Allow duplicate events in SyncHandleRegistry"

This reverts commit 327ed962ca0353e72e18a5eeeab5112d7d7058b6.

Reason for revert:
Suspected for breaking browser_tests failing on Win7 Tests (dbg)(1)
See  crbug.com/760381 

Original change's description:
> Mojo: Allow duplicate events in SyncHandleRegistry
> 
> Allows mulitple registrations for the same WaitableEvent in
> SyncHandleRegistry to allow for instances of independent nested waiters
> unwittingly waiting on the same event.
> 
> BUG=754945
> R=​yzshen@chromium.org
> 
> Change-Id: Ia166d860bf2a07a2db9dc39ae1350b8758950ff4
> Reviewed-on: https://chromium-review.googlesource.com/641402
> Reviewed-by: Yuzhu Shen <yzshen@chromium.org>
> Commit-Queue: Ken Rockot <rockot@chromium.org>
> Cr-Commit-Position: refs/heads/master@{#498246}

TBR=rockot@chromium.org,yzshen@chromium.org

Change-Id: I5f95fbcd9f3b602ccc16e381ca5f851445a55297
No-Presubmit: true
No-Tree-Checks: true
No-Try: true
Bug: 754945,  760381 
Reviewed-on: https://chromium-review.googlesource.com/642546
Reviewed-by: Tarun Bansal <tbansal@chromium.org>
Commit-Queue: Tarun Bansal <tbansal@chromium.org>
Cr-Commit-Position: refs/heads/master@{#498298}
[modify] https://crrev.com/0029db1595c245cbe93dd8b6f3039e9af625a452/ipc/ipc_sync_channel.cc
[modify] https://crrev.com/0029db1595c245cbe93dd8b6f3039e9af625a452/ipc/ipc_sync_message_filter.cc
[modify] https://crrev.com/0029db1595c245cbe93dd8b6f3039e9af625a452/mojo/public/cpp/bindings/lib/sync_event_watcher.cc
[modify] https://crrev.com/0029db1595c245cbe93dd8b6f3039e9af625a452/mojo/public/cpp/bindings/lib/sync_handle_registry.cc
[modify] https://crrev.com/0029db1595c245cbe93dd8b6f3039e9af625a452/mojo/public/cpp/bindings/sync_handle_registry.h

After revert, the next build looks green:
https://uberchromegw.corp.google.com/i/chromium.win/builders/Win7%20Tests%20%28dbg%29%281%29/builds/62768

Handing off the bug to rockot to track reland of the CL.

I'd consider this fixed by the revert. I don't think it makes sense to associate the reland with this bug.