Always mark entire prototype descriptor arrays.

BUG=chromium:506952
LOG=n

Review URL: https://codereview.chromium.org/1226783002

Cr-Commit-Position: refs/heads/master@{#29482}

src/heap/objects-visiting-inl.h

@@ -543,10 +543,12 @@ void StaticMarkingVisitor<StaticVisitor>::MarkMapContents(Heap* heap,
   }
 
   // Since descriptor arrays are potentially shared, ensure that only the
-  // descriptors that belong to this map are marked. The first time a
-  // non-empty descriptor array is marked, its header is also visited. The slot
-  // holding the descriptor array will be implicitly recorded when the pointer
-  // fields of this map are visited.
+  // descriptors that belong to this map are marked. The first time a non-empty
+  // descriptor array is marked, its header is also visited. The slot holding
+  // the descriptor array will be implicitly recorded when the pointer fields of
+  // this map are visited.  Prototype maps don't keep track of transitions, so
+  // just mark the entire descriptor array.
+  if (!map->is_prototype_map()) {
     DescriptorArray* descriptors = map->instance_descriptors();
     if (StaticVisitor::MarkObjectWithoutPush(heap, descriptors) &&
         descriptors->length() > 0) {
@@ -560,6 +562,7 @@ void StaticMarkingVisitor<StaticVisitor>::MarkMapContents(Heap* heap,
                                    descriptors->GetDescriptorStartSlot(start),
                                    descriptors->GetDescriptorEndSlot(end));
     }
+  }
 
   // Mark the pointer fields of the Map. Since the transitions array has
   // been marked already, it is fine that one of these fields contains a