PCScan: Make sure safepoint is entered iff the scanner thread is ready

Currently, in the initialization phase, if scheduling the PCScan thread
takes long time, the mutator can enter safepoint multiple times, causing
some internal data structures to be corrupted (traces). This change
makes sure that safepoints can only be entered if the scanning thread
had started execution.

Bug: 11297512
Change-Id: I0378ca4be1e0809833797003726f99de3253df53
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2822251


Commit-Queue: Anton Bikineev <bikineev@chromium.org>
Reviewed-by: Kentaro Hara <haraken@chromium.org>
Reviewed-by: Michael Lippautz <mlippautz@chromium.org>
Cr-Commit-Position: refs/heads/master@{#871967}