This CL removes the COMPLETE state from incremental marking. Since
then the only states left were STOPPED and MARKING, we can replace
the state with an is_running_ boolean field.

The state could change back-and-forth between MARKING and COMPLETE.
IsMarking() was already also checking for COMPLETE. So most code
already treated both states the same. IsComplete() now checks whether
marking is running and a transitive closure was reached already.

IncrementalMarking::Step() didn't process the marking queue when in
COMPLETE. This should be relatively rare though since it only
transitioned into COMPLETE when the stack guard was armed and the
allocation observer ran again before reaching a stack guard check.

Bug: v8:12775
Change-Id: Ied48d8c512ad3d1b3d2e29393d43b434b5fda8fe
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/3835689Reviewed-by: Michael Lippautz <mlippautz@chromium.org>
Commit-Queue: Dominik Inführ <dinfuehr@chromium.org>
Cr-Commit-Position: refs/heads/main@{#82581}