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When I looked at this code I convinced myself it's not used
on multiple threads. I'm wrong there. As we only care about
crashes on the main thread, this makes CrashIdHelper only
work on the main thread.

BUG=967947
TEST=none

Change-Id: Ic8e4c81012e31cff1f0ef771789a504de95d876b
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1633550Reviewed-by: Michael Wasserman <msw@chromium.org>
Commit-Queue: Scott Violet <sky@chromium.org>
Cr-Commit-Position: refs/heads/master@{#664355}