[heap] Do not clear mark bits of left trimmed old object start.

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 285 matching lines @@
 
   // IsBlackOrGrey assumes that the first bit is set for black or grey
   // objects.
   INLINE(static bool IsBlackOrGrey(MarkBit mark_bit)) { return mark_bit.Get(); }
 
   INLINE(static void MarkWhite(MarkBit markbit)) {
     markbit.Clear();
     markbit.Next().Clear();
   }
 
+  INLINE(static void MarkGrey(MarkBit markbit)) { markbit.Set(); }
+
+  INLINE(static void MarkBlack(MarkBit markbit)) {
+    markbit.Set();
+    markbit.Next().Set();
+  }
+
   INLINE(static void BlackToGrey(MarkBit markbit)) {
     DCHECK(IsBlack(markbit));
     markbit.Next().Clear();
   }
 
   INLINE(static void WhiteToGrey(MarkBit markbit)) {
     DCHECK(IsWhite(markbit));
     markbit.Set();
   }
 
@@ skipping 38 matching lines @@
   }
 
  private:
   DISALLOW_IMPLICIT_CONSTRUCTORS(Marking);
 };
 
 }  // namespace internal
 }  // namespace v8
 
 #endif  // V8_MARKING_H_