Introduce new state kCollectionStarted in CollectionBarrier. This state
is used during Heap::PerformGarbageCollection. It stops threads from
requesting GC when the GC was already started. This happens because a
background thread only requests the GC after it parked itself - the GC
could be started in-between those two events.

Bug: v8:10315
Change-Id: I59cf3d4ea41c7a2c37ffce89c5b057221a2499e0
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/2474858
Commit-Queue: Dominik Inführ <dinfuehr@chromium.org>
Reviewed-by: Ulan Degenbaev <ulan@chromium.org>
Cr-Commit-Position: refs/heads/master@{#70572}