X64: Fix bug in boolean conversion of empty string.

src/objects.h

 // Copyright 2006-2009 the V8 project authors. All rights reserved.
 // Redistribution and use in source and binary forms, with or without
 // modification, are permitted provided that the following conditions are
 // met:
 //
 //     * Redistributions of source code must retain the above copyright
 //       notice, this list of conditions and the following disclaimer.
 //     * Redistributions in binary form must reproduce the above
 //       copyright notice, this list of conditions and the following
 //       disclaimer in the documentation and/or other materials provided
@@ skipping 4020 matching lines @@
 
   // Casting.
   static inline SlicedString* cast(Object* obj);
 
   // Garbage collection support.
   void SlicedStringIterateBody(ObjectVisitor* v);
 
   // Layout description
 #if V8_HOST_ARCH_64_BIT
   // Optimizations expect buffer to be located at same offset as a ConsString's
-  // first substring. In 64 bit mode we have room for the size before the
-  // buffer.
+  // first substring. In 64 bit mode we have room for the start offset before
+  // the buffer.
   static const int kStartOffset = String::kSize;
   static const int kBufferOffset = kStartOffset + kIntSize;
   static const int kSize = kBufferOffset + kPointerSize;
 #else
   static const int kBufferOffset = String::kSize;
   static const int kStartOffset = kBufferOffset + kPointerSize;
   static const int kSize = kStartOffset + kIntSize;
 #endif
 
   // Support for StringInputBuffer.
@@ skipping 736 matching lines @@
     } else {
       value &= ~(1 << bit_position);
     }
     return value;
   }
 };
 
 } }  // namespace v8::internal
 
 #endif  // V8_OBJECTS_H_