[heap] Decouple stress-incremental-marking flag from concurrent-marking.

BUG=chromium:723600

Change-Id: Ic933159c973ccfe2a67fe8a4e2639c2ebd21b8d4
Reviewed-on: https://chromium-review.googlesource.com/563396Reviewed-by: Michael Lippautz <mlippautz@chromium.org>
Commit-Queue: Ulan Degenbaev <ulan@chromium.org>
Cr-Commit-Position: refs/heads/master@{#46471}

src/flag-definitions.h

@@ -719,7 +719,7 @@ DEFINE_BOOL(force_marking_deque_overflows, false,
 DEFINE_BOOL(stress_compaction, false,
             "stress the GC compactor to flush out bugs (implies "
             "--force_marking_deque_overflows)")
-DEFINE_BOOL(stress_incremental_marking, V8_CONCURRENT_MARKING_BOOL,
+DEFINE_BOOL(stress_incremental_marking, false,
             "force incremental marking for small heaps and run it more often")
 DEFINE_BOOL(manual_evacuation_candidates_selection, false,
             "Test mode only flag. It allows an unit test to select evacuation "