PCScan: Implement wait-free scanning

In the current safepoint synchronization scheme there is a global
barrier that ensures that safepoints are never left until scanning is
fully done. This causes all workers to block waiting for the slowest
one to finish and thereby causing all mutator threads to starve.

This CL introduces raceful wait-free scanning with offset based
randomization. This should provide a more sound forward progress
guarantee.

Bug: 11297512,1200485
Change-Id: I749f079a43be990abd7675598d7a141dbc94aea5
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2844590


Commit-Queue: Anton Bikineev <bikineev@chromium.org>
Reviewed-by: Michael Lippautz <mlippautz@chromium.org>
Cr-Commit-Position: refs/heads/master@{#875933}