Introduce lazy sweeping.

src/incremental-marking.h

Index: src/incremental-marking.h
diff --git a/src/incremental-marking.h b/src/incremental-marking.h
index 2681be017c30377da22c72daf4dcb0602f587e98..7bd99bf7dc0e324c63f4bfb5270b79bb1ed6a0db 100644
--- a/src/incremental-marking.h
+++ b/src/incremental-marking.h
@@ -42,6 +42,7 @@ class IncrementalMarking : public AllStatic {
  public:
   enum State {
     STOPPED,
+    SWEEPING,
     MARKING,
     COMPLETE
   };
@@ -57,7 +58,9 @@ class IncrementalMarking : public AllStatic {
 
   inline bool IsStopped() { return state() == STOPPED; }
 
-  inline bool IsMarking() { return state() == MARKING; }
+  inline bool IsMarking() { return state() >= MARKING; }
+
+  inline bool IsMarkingIncomplete() { return state() == MARKING; }
 
   bool WorthActivating();
 
@@ -73,6 +76,8 @@ class IncrementalMarking : public AllStatic {
 
   void Finalize();
 
+  void Abort();
+
   void MarkingComplete();
 
   // It's hard to know how much work the incremental marker should do to make
@@ -195,6 +200,16 @@ class IncrementalMarking : public AllStatic {
     allocation_marking_factor_ = kInitialAllocationMarkingFactor;
   }
 
+  static void ClearMarkbits(PagedSpace* space);
+  void ClearMarkbits();
+
+#ifdef DEBUG
+  void VerifyMarkbitsAreClean();
+  static void VerifyMarkbitsAreClean(PagedSpace* space);
+#endif
+
+  void StartMarking();
+
 
   Heap* heap_;