[heap] Enable black allocation when finalizing incremental marking.

BUG=chromium:630386

Review-Url: https://codereview.chromium.org/2224153003
Cr-Commit-Position: refs/heads/master@{#38487}

src/flag-definitions.h

@@ -738,7 +738,7 @@ DEFINE_INT(min_progress_during_incremental_marking_finalization, 32,
            "least this many unmarked objects")
 DEFINE_INT(max_incremental_marking_finalization_rounds, 3,
            "at most try this many times to finalize incremental marking")
-DEFINE_BOOL(black_allocation, false, "use black allocation")
+DEFINE_BOOL(black_allocation, true, "use black allocation")
 DEFINE_BOOL(concurrent_sweeping, true, "use concurrent sweeping")
 DEFINE_BOOL(parallel_compaction, true, "use parallel compaction")
 DEFINE_BOOL(parallel_pointer_update, true,