heap: Remove build flag for incremental marking

The flag has been defaulted on for some time now and we will only
disable incremental marking for debugging purposes, for which there's
still a runtime flag (HeapIncrementalMarking).

Change-Id: I4b71ae4a51cffb01f203163bc798cad34cc3088a
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1527529
Commit-Queue: Michael Lippautz <mlippautz@chromium.org>
Commit-Queue: Kentaro Hara <haraken@chromium.org>
Auto-Submit: Michael Lippautz <mlippautz@chromium.org>
Reviewed-by: Kentaro Hara <haraken@chromium.org>
Cr-Commit-Position: refs/heads/master@{#641568}
6 files changed