With parallel marking enabled, both visitors have to be equal wrt. to
actual visitation.

The differences are captured by template parameters:
- Retaining path tracing which we only do for full GCs.
- Incremental marking of FixedArray.

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:694255, chromium:750084
Change-Id: I177aeb0ee4f6a35e2f592ba257c9ddc14f88fd99
Reviewed-on: https://chromium-review.googlesource.com/704935
Commit-Queue: Michael Lippautz <mlippautz@chromium.org>
Reviewed-by: Ulan Degenbaev <ulan@chromium.org>
Cr-Commit-Position: refs/heads/master@{#48582}