Introduce lazy sweeping.

src/mark-compact.h

Index: src/mark-compact.h
diff --git a/src/mark-compact.h b/src/mark-compact.h
index 0cace03aef0c101760a91d517ad63bae97122565..19f13eac621d4e92ff603168f0e2e542a587986d 100644
--- a/src/mark-compact.h
+++ b/src/mark-compact.h
@@ -297,6 +297,16 @@ class MarkCompactCollector {
   inline bool is_code_flushing_enabled() const { return code_flusher_ != NULL; }
   void EnableCodeFlushing(bool enable);
 
+  enum SweeperType {
+    CONSERVATIVE,
+    LAZY_CONSERVATIVE,
+    PRECISE
+  };
+
+  // Sweep a single page from the given space conservatively.
+  // Return a number of reclaimed bytes.
+  static int SweepConservatively(PagedSpace* space, Page* p);
+
  private:
   MarkCompactCollector();
   ~MarkCompactCollector();
@@ -459,10 +469,10 @@ class MarkCompactCollector {
 
   void SweepNewSpace(NewSpace* space);
 
-  enum SweeperType { CONSERVATIVE, PRECISE };
 
   void SweepSpace(PagedSpace* space, SweeperType sweeper);
 
+
 #ifdef DEBUG
   // -----------------------------------------------------------------------
   // Debugging variables, functions and classes