Prototype of mark-and-compact support for Harmony weak maps.

src/mark-compact.h

Index: src/mark-compact.h
diff --git a/src/mark-compact.h b/src/mark-compact.h
index 179edba74e926a4ec8e65edc6c4449b413591e4d..8b5a41bb15658cc4b8e794cb34b4249e50629815 100644
--- a/src/mark-compact.h
+++ b/src/mark-compact.h
@@ -329,6 +329,16 @@ class MarkCompactCollector {
   // We replace them with a null descriptor, with the same key.
   void ClearNonLiveTransitions();
 
+  // Mark all values associated with reachable keys in weak maps encountered
+  // so far.  This might push new object or even new weak maps onto the
+  // marking stack.
+  void ProcessWeakMaps();
+
+  // After all reachable objects have been marked those weak map entries
+  // with an unreachable key are removed from all encountered weak maps.
+  // The linked list of all encountered weak maps is destroyed.
+  void ClearWeakMaps();
+
   // -----------------------------------------------------------------------
   // Phase 2: Sweeping to clear mark bits and free non-live objects for
   // a non-compacting collection, or else computing and encoding