[heap] Turn on black allocation.

BUG=

Review-Url: https://codereview.chromium.org/2589853002
Cr-Commit-Position: refs/heads/master@{#41815}

src/flag-definitions.h

@@ -754,7 +754,7 @@ DEFINE_INT(max_incremental_marking_finalization_rounds, 3,
            "at most try this many times to finalize incremental marking")
 DEFINE_BOOL(minor_mc, false, "perform young generation mark compact GCs")
 DEFINE_NEG_IMPLICATION(minor_mc, 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,