Reuse existing concurrent marking infrastructure for marking in the
atomic pause.

Details:
- Do not stop concurrent marking when entering the atomic pause.
- Start concurrent marking when it was off. This is required for
  non-incremental MC GCs.
- Use atomic marking state in MC.
- Transition through grey and avoid white->black.

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:750084
Change-Id: I9f84661e69beac6254494907bdfe94e865b1e3ab
Reviewed-on: https://chromium-review.googlesource.com/690002
Commit-Queue: Michael Lippautz <mlippautz@chromium.org>
Reviewed-by: Ulan Degenbaev <ulan@chromium.org>
Cr-Commit-Position: refs/heads/master@{#48281}