Small fixes to hash seeding code, discovered during

src/objects.h

Index: src/objects.h
===================================================================
--- src/objects.h	(revision 10367)
+++ src/objects.h	(working copy)
@@ -2604,7 +2604,7 @@
   }
   static uint32_t HashForObject(Key key, Object* object) { return 0; }
   static uint32_t SeededHashForObject(Key key, uint32_t seed, Object* object) {
-    // Won't be called if UsesSeed isn't overridden by child class.
+    ASSERT(UsesSeed);
     return HashForObject(key, object);
   }
 };