Assert that the lock is help whenever AssignUnsafe is called.
LazyMutex::AssertHeld resolves to a DCHECK so this should not
regress production performance (other than the call itself
that might not be inlined).

Bug: chromium:1056170
Change-Id: Ic2005d180e6960c24dff7743aa3e0d5e57a63d80
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/2716286Reviewed-by: Michael Lippautz <mlippautz@chromium.org>
Commit-Queue: Omer Katz <omerkatz@chromium.org>
Cr-Commit-Position: refs/heads/master@{#73000}