I believe this is related to 

https://cs.chromium.org/chromium/src/ui/events/blink/compositor_thread_event_queue.cc?sq=package:chromium&l=66

and
https://cs.chromium.org/chromium/src/ui/events/blink/compositor_thread_event_queue.cc?sq=package:chromium&l=47

These set_coalesced calls make no sense to me. I think we should remove them will try to debug on Monday.

The following revision refers to this bug:
  https://chromium.googlesource.com/chromium/src.git/+/4d369911c9ff7aa9444516179120e727da3641d4

commit 4d369911c9ff7aa9444516179120e727da3641d4
Author: Dave Tapuska <dtapuska@chromium.org>
Date: Tue Oct 31 13:25:47 2017

Vsync aligned gesture events setting coalesced bit.

Do not set the coalesced bit on vsync aligned gestures. These are the
original latency_info objects so they aren't coalesced the other ones
are set as coalesced in the event queue. This code caused DCHECKs to fire
in the browser once the coalesced bit was propagated correctly.

BUG= 779280 

Change-Id: Ia873f4ebd5b4cb22fcf40033775cf8618f80b959
Reviewed-on: https://chromium-review.googlesource.com/744801
Reviewed-by: Chong Zhang <chongz@chromium.org>
Commit-Queue: Dave Tapuska <dtapuska@chromium.org>
Cr-Commit-Position: refs/heads/master@{#512806}
[modify] https://crrev.com/4d369911c9ff7aa9444516179120e727da3641d4/ui/events/blink/compositor_thread_event_queue.cc

 Issue 780274  has been merged into this issue.