The {baseline_compilation_units_} and {tiering_compilation_units_}
fields should only be accessed if the {mutex_} is held.
Also, the number of compilation units is already taken care of inside
of {RestartBackgroundTasks}, so no need to explicitly pass it.

R=ahaas@chromium.org

Change-Id: I8f36ed141b587ee1bea41291545f39546d8cf24e
Reviewed-on: https://chromium-review.googlesource.com/1044213Reviewed-by: Andreas Haas <ahaas@chromium.org>
Commit-Queue: Clemens Hammacher <clemensh@chromium.org>
Cr-Commit-Position: refs/heads/master@{#52994}