CompilationState had three different mutexes, plus two atomic fields.
Not holding the right mutexes at the right time has already led to
failures. Hence, only use a single mutex to protect all shared state of
the CompilationState.

R=ahaas@chromium.org

Bug: chromium:824681
Change-Id: I2c414f3ddb75e82944621590493fadcbbdfb781c
Reviewed-on: https://chromium-review.googlesource.com/1000783Reviewed-by: Andreas Haas <ahaas@chromium.org>
Commit-Queue: Clemens Hammacher <clemensh@chromium.org>
Cr-Commit-Position: refs/heads/master@{#52481}