Add map-based read barrier to WeakCell

include/v8.h

 // Copyright 2012 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.
 
 /** \mainpage V8 API Reference Guide
  *
  * V8 is Google's open source JavaScript engine.
  *
  * This set of documents provides reference material generated from the
  * V8 header file, include/v8.h.
@@ skipping 6197 matching lines @@
       4 * kApiPointerSize;
   static const int kAmountOfExternalAllocatedMemoryAtLastGlobalGCOffset =
       kAmountOfExternalAllocatedMemoryOffset + kApiInt64Size;
   static const int kIsolateRootsOffset =
       kAmountOfExternalAllocatedMemoryAtLastGlobalGCOffset + kApiInt64Size +
       kApiPointerSize;
   static const int kUndefinedValueRootIndex = 5;
   static const int kNullValueRootIndex = 7;
   static const int kTrueValueRootIndex = 8;
   static const int kFalseValueRootIndex = 9;
-  static const int kEmptyStringRootIndex = 156;
+  static const int kEmptyStringRootIndex = 157;
 
   // The external allocation limit should be below 256 MB on all architectures
   // to avoid that resource-constrained embedders run low on memory.
   static const int kExternalAllocationLimit = 192 * 1024 * 1024;
 
   static const int kNodeClassIdOffset = 1 * kApiPointerSize;
   static const int kNodeFlagsOffset = 1 * kApiPointerSize + 3;
   static const int kNodeStateMask = 0x7;
   static const int kNodeStateIsWeakValue = 2;
   static const int kNodeStateIsPendingValue = 3;
@@ skipping 1273 matching lines @@
  */
 
 
 }  // namespace v8
 
 
 #undef TYPE_CHECK
 
 
 #endif  // V8_H_