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

src/json-stringifier.h

Index: src/json-stringifier.h
diff --git a/src/json-stringifier.h b/src/json-stringifier.h
index 93709e6967349bb6ff4b6e57b2a71fbd0de26574..36697828745fc1f31ec7228fb245d570eaa0cea6 100644
--- a/src/json-stringifier.h
+++ b/src/json-stringifier.h
@@ -13,11 +13,12 @@ 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);
@@ -25,6 +26,8 @@ class BasicJsonStringifier BASE_EMBEDDED {
  private:
   enum Result { UNCHANGED, SUCCESS, EXCEPTION };
 
+  bool InitializeGap(Handle<Object> gap);
+
   MUST_USE_RESULT MaybeHandle<Object> ApplyToJsonFunction(
       Handle<Object> object,
       Handle<Object> key);
@@ -104,7 +107,6 @@ class BasicJsonStringifier BASE_EMBEDDED {
   IncrementalStringBuilder builder_;
   Handle<String> tojson_string_;
   Handle<JSArray> stack_;
-  Handle<String> gap_string_;
   uc16* gap_;
   int indent_;