Fix invariant so that we cannot record relocation slots for

src/incremental-marking-inl.h

Index: src/incremental-marking-inl.h
===================================================================
--- src/incremental-marking-inl.h	(revision 12482)
+++ src/incremental-marking-inl.h	(working copy)
@@ -48,7 +48,9 @@
     // Object is either grey or white.  It will be scanned if survives.
     return false;
   }
-  return true;
+  if (!is_compacting_) return false;
+  MarkBit obj_bit = Marking::MarkBitFrom(obj);
+  return Marking::IsBlack(obj_bit);
 }