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This may enable us to remove support for recursive mutexes in the near
future.

Bugs in this CL are likely to manifest as threads hanging while
trying to acquire this mutex recursively (while manipulating GC objects).

Bug: 856641
Change-Id: Icfc3982f7489fb53ddd4df319c405002ee1c3ea8
Reviewed-on: https://chromium-review.googlesource.com/1112197Reviewed-by: Kentaro Hara <haraken@chromium.org>
Commit-Queue: Jeremy Roman <jbroman@chromium.org>
Cr-Commit-Position: refs/heads/master@{#570884}