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During marking, (a) marking an object and (b) processing it currently
happen on the main thread. In future, concurrent marking will move both
steps to helper tasks.

Even with concurrent marking, the write barrier remains on the main
thread and will require at least (a) to be executed right away. It is
thus beneficial to increment live bytes counters in (b) which can be
offloaded to helper tasks.

Output on Z840 (down from ~13.1x):
[ RUN      ] WriteBarrierPerfTest.MemberWritePerformance
*RESULT WriteBarrierPerfTest writes during GC: = 46497.21016738996 writes/ms
*RESULT WriteBarrierPerfTest writes outside GC: = 541516.2454873646 writes/ms
*RESULT WriteBarrierPerfTest relative speed difference: = 11.646209386281587 times
[       OK ] WriteBarrierPerfTest.MemberWritePerformance (67 ms)

Bug: 1014414
Change-Id: I5295e4738fce9410fbfbed51829c04b2e25f2d9e
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1883719
Commit-Queue: Michael Lippautz <mlippautz@chromium.org>
Reviewed-by: Kentaro Hara <haraken@chromium.org>
Reviewed-by: Omer Katz <omerkatz@chromium.org>
Cr-Commit-Position: refs/heads/master@{#710262}