Implement HeapIterator that skips over unreachable objects.

src/heap.h

Index: src/heap.h
diff --git a/src/heap.h b/src/heap.h
index fbcc70df53155153df896c7383c3bbf21de83899..bbda22e57051dd9612b533eed5986fadda92cb42 100644
--- a/src/heap.h
+++ b/src/heap.h
@@ -1585,17 +1585,18 @@ class SpaceIterator : public Malloced {
 // nodes filtering uses GC marks, it can't be used during MS/MC GC
 // phases. Also, it is forbidden to interrupt iteration in this mode,
 // as this will leave heap objects marked (and thus, unusable).
-class FreeListNodesFilter;
+class HeapObjectsFilter;
 
 class HeapIterator BASE_EMBEDDED {
  public:
-  enum FreeListNodesFiltering {
+  enum HeapObjectsFiltering {
     kNoFiltering,
-    kPreciseFiltering
+    kFilterFreeListNodes,
+    kFilterUnreachable
   };
 
   HeapIterator();
-  explicit HeapIterator(FreeListNodesFiltering filtering);
+  explicit HeapIterator(HeapObjectsFiltering filtering);
   ~HeapIterator();
 
   HeapObject* next();
@@ -1608,8 +1609,8 @@ class HeapIterator BASE_EMBEDDED {
   void Shutdown();
   HeapObject* NextObject();
 
-  FreeListNodesFiltering filtering_;
-  FreeListNodesFilter* filter_;
+  HeapObjectsFiltering filtering_;
+  HeapObjectsFilter* filter_;
   // Space iterator for iterating all the spaces.
   SpaceIterator* space_iterator_;
   // Object iterator for the space currently being iterated.