Mark faster when executing write barriers.

src/incremental-marking.h

Index: src/incremental-marking.h
diff --git a/src/incremental-marking.h b/src/incremental-marking.h
index d47c300ef3f97893e3f018c0aab3a20c76856da9..a4dd5f33140332acb78742ff074825ad2bc920d9 100644
--- a/src/incremental-marking.h
+++ b/src/incremental-marking.h
@@ -100,7 +100,7 @@ class IncrementalMarking {
   // Do some marking every time this much memory has been allocated or that many
   // heavy (color-checking) write barriers have been invoked.
   static const intptr_t kAllocatedThreshold = 65536;
-  static const intptr_t kWriteBarriersInvokedThreshold = 65536;
+  static const intptr_t kWriteBarriersInvokedThreshold = 32768;
   // Start off by marking this many times more memory than has been allocated.
   static const intptr_t kInitialMarkingSpeed = 1;
   // But if we are promoting a lot of data we need to mark faster to keep up