Fix slow path of JSON.stringifier when GC strikes.

src/json-stringifier.h

 // Copyright 2012 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 277 matching lines @@
   static const int kJsonQuoteWorstCaseBlowup = 6;
   static const int kSpaceForQuotes = 2;
   int worst_case_length =
       object->length() * kJsonQuoteWorstCaseBlowup + kSpaceForQuotes;
 
   if (worst_case_length > 32 * KB) {  // Slow path if too large.
     BasicJsonStringifier stringifier(isolate);
     return stringifier.Stringify(object);
   }
 
-  FlattenString(object);
-  String::FlatContent flat = object->GetFlatContent();
-  if (flat.IsAscii()) {
+  object = FlattenGetString(object);
+  ASSERT(object->IsFlat());
+  if (object->IsOneByteRepresentation()) {
+    Handle<String> result =
+        isolate->factory()->NewRawOneByteString(worst_case_length);
+    AssertNoAllocation no_alloc;
+    const uint8_t* start = object->IsSeqOneByteString()
+        ? SeqOneByteString::cast(*object)->GetChars()
+        : ExternalAsciiString::cast(*object)->GetChars();
     return StringifyString_<SeqOneByteString>(
         isolate,
-        flat.ToOneByteVector(),
-        isolate->factory()->NewRawOneByteString(worst_case_length));
+        Vector<const uint8_t>(start, object->length()),
+        result);
   } else {
-    ASSERT(flat.IsTwoByte());
+    Handle<String> result =
+        isolate->factory()->NewRawTwoByteString(worst_case_length);
+    AssertNoAllocation no_alloc;
+    const uc16* start = object->IsSeqTwoByteString()
+        ? SeqTwoByteString::cast(*object)->GetChars()
+        : ExternalTwoByteString::cast(*object)->GetChars();
     return StringifyString_<SeqTwoByteString>(
         isolate,
-        flat.ToUC16Vector(),
-        isolate->factory()->NewRawTwoByteString(worst_case_length));
+        Vector<const uc16>(start, object->length()),
+        result);
   }
 }
 
 
 template <typename ResultType, typename Char>
 MaybeObject* BasicJsonStringifier::StringifyString_(Isolate* isolate,
                                                     Vector<Char> vector,
                                                     Handle<String> result) {
   AssertNoAllocation no_allocation;
   int final_size = 0;
@@ skipping 513 matching lines @@
       SerializeString_<false, uint8_t>(object);
     } else {
       SerializeString_<false, uc16>(object);
     }
   }
 }
 
 } }  // namespace v8::internal
 
 #endif  // V8_JSON_STRINGIFIER_H_