The regression sneaked in when unifying the marking visitors in
4b42656d. The concurrent marker keeps a
local live byte count per page so it is safe to rely on non-atomic
writes from the main thread.

CQ_INCLUDE_TRYBOTS=master.tryserver.v8:v8_linux64_tsan_rel;master.tryserver.v8:v8_linux64_tsan_concurrent_marking_rel_ng;master.tryserver.blink:linux_trusty_blink_rel;master.tryserver.chromium.linux:linux_optional_gpu_tests_rel;master.tryserver.chromium.mac:mac_optional_gpu_tests_rel;master.tryserver.chromium.win:win_optional_gpu_tests_rel;master.tryserver.chromium.android:android_optional_gpu_tests_rel

Bug: chromium:775864
Change-Id: If67d2accd1d7953aa30d815da8bf41b0039ad2dd
Reviewed-on: https://chromium-review.googlesource.com/728239
Commit-Queue: Michael Lippautz <mlippautz@chromium.org>
Reviewed-by: Ulan Degenbaev <ulan@chromium.org>
Cr-Commit-Position: refs/heads/master@{#48758}