[json] Repair JSON.parse regression with non-sequential strings.

Make sure to flatten strings first in JSON.parse() builtins, otherwise
we always hit the slow path for non-sequential strings, i.e. for cons
strings.

Also don't create any arguments adaptor frames for JSON.parse() as the
C++ builtin can handle any number of inputs properly.

R=yangguo@chromium.org

Review-Url: https://codereview.chromium.org/2039553002
Cr-Commit-Position: refs/heads/master@{#36722}

src/bootstrapper.cc

@@ -1603,7 +1603,7 @@ void Genesis::InitializeGlobal(Handle<JSGlobalObject> global_object,
     Handle<JSObject> json_object = factory->NewJSObject(cons, TENURED);
     DCHECK(json_object->IsJSObject());
     JSObject::AddProperty(global, name, json_object, DONT_ENUM);
-    SimpleInstallFunction(json_object, "parse", Builtins::kJsonParse, 2, true);
+    SimpleInstallFunction(json_object, "parse", Builtins::kJsonParse, 2, false);
     SimpleInstallFunction(json_object, "stringify", Builtins::kJsonStringify, 3,
                           true);
     JSObject::AddProperty(

src/builtins.cc

@@ -2251,6 +2251,7 @@ BUILTIN(JsonParse) {
   Handle<String> string;
   ASSIGN_RETURN_FAILURE_ON_EXCEPTION(isolate, string,
                                      Object::ToString(isolate, source));
+  string = String::Flatten(string);
   RETURN_RESULT_OR_FAILURE(
       isolate, string->IsSeqOneByteString()
                    ? JsonParser<true>::Parse(isolate, string, reviver)