[json] implement JSON.stringify gap pre-processing in C++.

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/objects.h"
 #include "src/string-builder.h"
 
 namespace v8 {
 namespace internal {
 
 class BasicJsonStringifier BASE_EMBEDDED {
  public:
-  BasicJsonStringifier(Isolate* isolate, Handle<String> gap);
+  explicit BasicJsonStringifier(Isolate* isolate);
 
   ~BasicJsonStringifier() { DeleteArray(gap_); }
 
-  MUST_USE_RESULT MaybeHandle<Object> Stringify(Handle<Object> object);
+  MUST_USE_RESULT MaybeHandle<Object> Stringify(Handle<Object> object,
+                                                Handle<Object> gap);
 
   MUST_USE_RESULT static MaybeHandle<Object> StringifyString(
       Isolate* isolate, Handle<String> object);
 
  private:
   enum Result { UNCHANGED, SUCCESS, EXCEPTION };
 
+  bool InitializeGap(Handle<Object> gap);
+
   MUST_USE_RESULT MaybeHandle<Object> ApplyToJsonFunction(
       Handle<Object> object,
       Handle<Object> key);
 
   // Entry point to serialize the object.
   INLINE(Result SerializeObject(Handle<Object> obj)) {
     return Serialize_<false>(obj, false, factory()->empty_string());
   }
 
   // Serialize an array element.
@@ skipping 59 matching lines @@
 
   Result StackPush(Handle<Object> object);
   void StackPop();
 
   Factory* factory() { return isolate_->factory(); }
 
   Isolate* isolate_;
   IncrementalStringBuilder builder_;
   Handle<String> tojson_string_;
   Handle<JSArray> stack_;
-  Handle<String> gap_string_;
   uc16* gap_;
   int indent_;
 
   static const int kJsonEscapeTableEntrySize = 8;
   static const char* const JsonEscapeTable;
 };
 
 }  // namespace internal
 }  // namespace v8
 
 #endif  // V8_JSON_STRINGIFIER_H_