Don't try to get the mark bits of the one ptr filler object

BUG=chromium:???
LOG=n
R=mlippautz@chromium.org,hpayer@chromium.org

Review URL: https://codereview.chromium.org/1252333002

Cr-Commit-Position: refs/heads/master@{#29850}

src/heap/heap.cc

@@ -6277,6 +6277,7 @@ class UnreachableObjectsFilter : public HeapObjectsFilter {
   }
 
   bool SkipObject(HeapObject* object) {
+    if (object->IsFiller()) return true;
     MarkBit mark_bit = Marking::MarkBitFrom(object);
     return Marking::IsWhite(mark_bit);
   }