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This frees up the regular HeapIncrementalMarking flag for implementing an
initial incremental marking strategy.

Bug: chromium:757440
Change-Id: Iee924ef718f450b4d1e865ef6880242f501376fb
Reviewed-on: https://chromium-review.googlesource.com/1028273Reviewed-by: John Budorick <jbudorick@chromium.org>
Reviewed-by: Kentaro Hara <haraken@chromium.org>
Reviewed-by: Keishi Hattori <keishi@chromium.org>
Commit-Queue: Michael Lippautz <mlippautz@chromium.org>
Cr-Commit-Position: refs/heads/master@{#553935}