[heap] Remove ObjectMarking::BlackToWhite and ObjectMarking::GreyToWhite.

src/heap/marking.h

 // Copyright 2016 the V8 project authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 #ifndef V8_MARKING_H
 #define V8_MARKING_H
 
 #include "src/utils.h"
 
 namespace v8 {
@@ skipping 294 matching lines @@
 
   INLINE(static void BlackToWhite(MarkBit markbit)) {
     DCHECK(IsBlack(markbit));
     markbit.Clear();
     markbit.Next().Clear();
   }
 
   INLINE(static void GreyToWhite(MarkBit markbit)) {
     DCHECK(IsGrey(markbit));
     markbit.Clear();
-    markbit.Next().Clear();
   }
 
   INLINE(static void BlackToGrey(MarkBit markbit)) {
     DCHECK(IsBlack(markbit));
     markbit.Next().Clear();
   }
 
   INLINE(static void WhiteToGrey(MarkBit markbit)) {
     DCHECK(IsWhite(markbit));
     markbit.Set();
@@ skipping 40 matching lines @@
   }
 
  private:
   DISALLOW_IMPLICIT_CONSTRUCTORS(Marking);
 };
 
 }  // namespace internal
 }  // namespace v8
 
 #endif  // V8_MARKING_H_