Heap::UnregisterStrongRoots needs to iterate the list of all strong
roots to delete the given slot. This CL changes
Heap::RegisterStrongRoots to return the pointer to the linked list node.
Heap::UnregisterStrongRoots gets the node as argument and can directly
delete it in constant time.

The CL also introduces Heap::UpdateStrongRoots which can update a
node without locking the mutex.

Bug: v8:10315
Change-Id: I2c021517c010a659821f8c10de758bb49b28449f
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/2364511Reviewed-by: Ulan Degenbaev <ulan@chromium.org>
Reviewed-by: Jakob Gruber <jgruber@chromium.org>
Commit-Queue: Dominik Inführ <dinfuehr@chromium.org>
Cr-Commit-Position: refs/heads/master@{#69499}