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This CL resolves a race where multiple threads can access the default policy
host restrictions. This can lead to crashes in the wild. Fix this by controlling
access to the default policy host restictions via a global lock.

BUG=978407

Change-Id: Ic5d5f4e42980017f4e0b4d56b1f4d9f79983223a
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1700323
Commit-Queue: Karan Bhatia <karandeepb@chromium.org>
Reviewed-by: Devlin <rdevlin.cronin@chromium.org>
Cr-Commit-Position: refs/heads/master@{#677113}