Use FlatStringReader in JSON stringifier.

src/json-stringifier.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.
 
 #ifndef V8_JSON_STRINGIFIER_H_
 #define V8_JSON_STRINGIFIER_H_
 
 #include "src/v8.h"
 
 #include "src/conversions.h"
@@ skipping 621 matching lines @@
   // an escaped character is 6.  Shifting the remainin string length right by 3
   // is a more pessimistic estimate, but faster to calculate.
   int worst_case_length = length << 3;
   if (builder_.CurrentPartCanFit(worst_case_length)) {
     DisallowHeapAllocation no_gc;
     Vector<const SrcChar> vector = GetCharVector<SrcChar>(string);
     IncrementalStringBuilder::NoExtendBuilder<DestChar> no_extend(
         &builder_, worst_case_length);
     SerializeStringUnchecked_(vector, &no_extend);
   } else {
-    String* string_location = NULL;
-    Vector<const SrcChar> vector(NULL, 0);
-    for (int i = 0; i < length; i++) {
-      // If GC moved the string, we need to refresh the vector.
-      if (*string != string_location) {
-        DisallowHeapAllocation no_gc;
-        // This does not actually prevent the string from being relocated later.
-        vector = GetCharVector<SrcChar>(string);
-        string_location = *string;
-      }
-      SrcChar c = vector[i];
+    FlatStringReader reader(isolate_, string);
+    for (int i = 0; i < reader.length(); i++) {
+      SrcChar c = static_cast<SrcChar>(reader.Get(i));
       if (DoNotEscape(c)) {
         builder_.Append<SrcChar, DestChar>(c);
       } else {
         builder_.AppendCString(&JsonEscapeTable[c * kJsonEscapeTableEntrySize]);
       }
     }
   }
 
   builder_.Append<uint8_t, DestChar>('"');
 }
@@ skipping 42 matching lines @@
       SerializeString_<uint8_t, uc16>(object);
     } else {
       SerializeString_<uc16, uc16>(object);
     }
   }
 }
 
 } }  // namespace v8::internal
 
 #endif  // V8_JSON_STRINGIFIER_H_