X64: Fix bug in boolean conversion of empty string.

src/objects.h

Index: src/objects.h
diff --git a/src/objects.h b/src/objects.h
index d367f815fd8aaf8f5ab5be7bf571a375faeecb6e..b0d9d87099e70738d254a91431ca18363d4ea70f 100644
--- a/src/objects.h
+++ b/src/objects.h
@@ -4038,8 +4038,8 @@ class SlicedString: public String {
   // 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;