CanonicalHandleScope is now also used on background threads. Therefore
Heap::RegisterStrongRoots and Heap::UnregisterStrongRoots are not
exclusively used on the main thread anymore. Simply protect this list
with a mutex.

Bug: v8:10315, v8:10814
Change-Id: Id08269c9f7fecae8c570ab711c522d111b06b005
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/2364503Reviewed-by: Ulan Degenbaev <ulan@chromium.org>
Commit-Queue: Dominik Inführ <dinfuehr@chromium.org>
Cr-Commit-Position: refs/heads/master@{#69488}