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Add a general counter for threads that are currently in incremental
marking.

This guard can be checked as first reason for bailing out in any write
barrier as it only requires a single load and does not need to go
through TLS or page lookups.

Bug: chromium:757440
Change-Id: Ida63e560bb52ccd66da739139c0510229b05a550
Reviewed-on: https://chromium-review.googlesource.com/995272
Commit-Queue: Michael Lippautz <mlippautz@chromium.org>
Reviewed-by: Kentaro Hara <haraken@chromium.org>
Cr-Commit-Position: refs/heads/master@{#548033}