Start incremental marking on idle notification.

src/incremental-marking.h

Index: src/incremental-marking.h
diff --git a/src/incremental-marking.h b/src/incremental-marking.h
index b5d9f1b9b733a8b460159c37d13a3bd2ce2df8ab..25def8706867528565304500a460c332fec2c47b 100644
--- a/src/incremental-marking.h
+++ b/src/incremental-marking.h
@@ -63,6 +63,8 @@ class IncrementalMarking {
 
   inline bool IsMarkingIncomplete() { return state() == MARKING; }
 
+  inline bool IsComplete() { return state() == COMPLETE; }
+
   bool WorthActivating();
 
   void Start();
@@ -101,6 +103,7 @@ class IncrementalMarking {
   void OldSpaceStep(intptr_t allocated) {
     Step(allocated * kFastMarking / kInitialAllocationMarkingFactor);
   }
+
   void Step(intptr_t allocated);
 
   inline void RestartIfNotMarking() {