Callbacks can be called and deleted from any thread, so they need to be
protected by a mutex. The deleted comment in {NotifyOnEvent} is
outdated.
Use a separate mutex such that callbacks can call back into the
NativeModule or CompilationState without deadlocking.

R=ahaas@chromium.org

Bug: v8:8904, v8:8689
Change-Id: If28a1f5682894518453b216c3ea152e5d6d8afdb
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/1505457Reviewed-by: Andreas Haas <ahaas@chromium.org>
Commit-Queue: Clemens Hammacher <clemensh@chromium.org>
Cr-Commit-Position: refs/heads/master@{#60065}