Fix migration checks and extend them to the Scavenger.

src/heap.h

Index: src/heap.h
diff --git a/src/heap.h b/src/heap.h
index e0ffa63e9abdf8678925b12cef99b154ef76163a..3472ec09328e1278b8cfbfe95ca2f03e9d3d0571 100644
--- a/src/heap.h
+++ b/src/heap.h
@@ -1391,6 +1391,10 @@ class Heap {
   inline OldSpace* TargetSpace(HeapObject* object);
   static inline AllocationSpace TargetSpaceId(InstanceType type);
 
+  // Checks whether the given object is allowed to be migrated from it's
+  // current space into the given destination space. Used for debugging.
+  inline bool AllowedToBeMigrated(HeapObject* object, AllocationSpace dest);
+
   // Sets the stub_cache_ (only used when expanding the dictionary).
   void public_set_code_stubs(UnseededNumberDictionary* value) {
     roots_[kCodeStubsRootIndex] = value;