CollectionRequested was used exclusively on the main thread when a
background thread requested a GC. The main thread never used
SafepointRequested at any time. Now with the shared GC we might need to
stop multiple isolates in a safepoint in the future. In such a situation
we would need to use SafepointRequested also on the main thread.

This CL prepares V8 for this situation by using SafepointRequested
instead of CollectionRequested and friends on the main thread. The slow
path of Safepoint(), Park() and Unpark() will check in the future
whether the main thread needs to halt for a shared GC or needs to
perform a local GC. At the moment, simply performing the local GC is
still enough.

Bug: v8:11708
Change-Id: I819b6f7db8251074a4adf8b554e0a1393c76f7da
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/2891834
Commit-Queue: Dominik Inführ <dinfuehr@chromium.org>
Reviewed-by: Michael Lippautz <mlippautz@chromium.org>
Cr-Commit-Position: refs/heads/master@{#74673}