This has happened again to me. Let me know if there is anything I can to do help figure this out.

Fix posted https://chromium-review.googlesource.com/c/584554/

Sorry for the productivity loss, but thanks for the bug and stacktrace!

Whoops, wrong link, Should have been https://chromium-review.googlesource.com/c/585332/ ... I'm having a day.

The following revision refers to this bug:
  https://chromium.googlesource.com/chromium/src.git/+/b99830cc8f7df9632fc949daa233a78f824de61d

commit b99830cc8f7df9632fc949daa233a78f824de61d
Author: Sky Malice <skym@chromium.org>
Date: Tue Jul 25 21:10:32 2017

[Sync] Fix iterator error in UserEventSyncBridge::HandleGlobalIdChange.

There were two mistakes in the previous code, and together they were
able to cause an infinite loop.

The first, and most egregious, was the iterating from
std::multimap::find() to multimap::end(). The intention was to only
iterate over the pairs with a given key, but instead all pairs were
iterated over with a key equal or greater. This did trigger the
DCHECK when we iterate onto an element with the wrong navigation_id,
but if DCHECKs were not on these events would incorrectly be
processed.

The second, and arguably not a bug if the former was working
correctly, was while this iter loop was happening, which deleted
items, we would call RecordUserEvent which would insert a new pair
into in_flight_nav_linked_events_ which we were iterating over. If
the new element had a key greater than or equal to our current iter
then we would encounter it. And thus we infinitely loop, erasing and
inserting what was essentially the same element over and over.

This fix removes both of these issues. multimap::equal_range() for
the former, and a temp vector for the latter.

Bug:  747621 ,  748600 
Change-Id: I2a48ee1f86c81eec787dfbf293ebabc239f11e5d
Reviewed-on: https://chromium-review.googlesource.com/585332
Reviewed-by: Patrick Noland <pnoland@chromium.org>
Commit-Queue: Sky Malice <skym@chromium.org>
Cr-Commit-Position: refs/heads/master@{#489432}
[modify] https://crrev.com/b99830cc8f7df9632fc949daa233a78f824de61d/components/sync/user_events/user_event_sync_bridge.cc
[modify] https://crrev.com/b99830cc8f7df9632fc949daa233a78f824de61d/components/sync/user_events/user_event_sync_bridge_unittest.cc