Implement map collection for incremental marking.

src/mark-compact.h

Index: src/mark-compact.h
diff --git a/src/mark-compact.h b/src/mark-compact.h
index 6420a21fe3b0ac746706bf13605188c80e1bf83e..8bee6ab9dcab0e546d4e84f8ca75e7b1bdfeaf71 100644
--- a/src/mark-compact.h
+++ b/src/mark-compact.h
@@ -584,7 +584,7 @@ class MarkCompactCollector {
 
   bool was_marked_incrementally_;
 
-  bool collect_maps_;
+  bool clear_map_transitions_;
 
   bool flush_monomorphic_ics_;
 
@@ -608,12 +608,12 @@ class MarkCompactCollector {
   //
   //   After: Live objects are marked and non-live objects are unmarked.
 
-
   friend class RootMarkingVisitor;
   friend class MarkingVisitor;
   friend class StaticMarkingVisitor;
   friend class CodeMarkingVisitor;
   friend class SharedFunctionInfoMarkingVisitor;
+  friend class IncrementalMarking;
 
   // Mark non-optimize code for functions inlined into the given optimized
   // code. This will prevent it from being flushed.
@@ -642,8 +642,9 @@ class MarkCompactCollector {
 
   void ProcessNewlyMarkedObject(HeapObject* obj);
 
-  // Mark a Map and its DescriptorArray together, skipping transitions.
-  void MarkMapContents(Map* map);
+  // Mark weak pointers in a Map and its DescriptorArray together, possibly
+  // skipping transitions or back pointers.
+  void MarkWeakMapContents(Map* map);
   void MarkAccessorPairSlot(HeapObject* accessors, int offset);
   void MarkDescriptorArray(DescriptorArray* descriptors);