Bug 1656359 - eliminate needless string flattening when calling JS_ParseJSON; r=peterv

JS_ParseJSON doesn't require a null-terminated buffer, so we shouldn't have
to provide one.

Differential Revision: https://phabricator.services.mozilla.com/D85475

dom/bindings/BindingUtils.cpp

@@ -2115,8 +2115,7 @@ bool DictionaryBase::ParseJSON(JSContext* aCx, const nsAString& aJSON,
   if (aJSON.IsEmpty()) {
     return true;
   }
-  return JS_ParseJSON(aCx, PromiseFlatString(aJSON).get(), aJSON.Length(),
-                      aVal);
+  return JS_ParseJSON(aCx, aJSON.BeginReading(), aJSON.Length(), aVal);
 }
 
 bool DictionaryBase::StringifyToJSON(JSContext* aCx, JS::Handle<JSObject*> aObj,

dom/payments/PaymentRequestUtils.cpp

@@ -50,9 +50,7 @@ nsresult DeserializeToJSObject(const nsAString& aSerializedObject,
 nsresult DeserializeToJSValue(const nsAString& aSerializedObject,
                               JSContext* aCx, JS::MutableHandleValue aValue) {
   MOZ_ASSERT(aCx);
-  if (!JS_ParseJSON(aCx,
-                    static_cast<const char16_t*>(
-                        PromiseFlatString(aSerializedObject).get()),
+  if (!JS_ParseJSON(aCx, aSerializedObject.BeginReading(),
                     aSerializedObject.Length(), aValue)) {
     return NS_ERROR_UNEXPECTED;
   }

dom/reporting/ReportingHeader.cpp

@@ -225,8 +225,7 @@ void ReportingHeader::ReportingFromChannel(nsIHttpChannel* aChannel) {
 
   JSContext* cx = jsapi.cx();
   JS::Rooted<JS::Value> jsonValue(cx);
-  bool ok = JS_ParseJSON(cx, PromiseFlatString(json).get(), json.Length(),
-                         &jsonValue);
+  bool ok = JS_ParseJSON(cx, json.BeginReading(), json.Length(), &jsonValue);
   if (!ok) {
     LogToConsoleInvalidJSON(aChannel, aURI);
     return nullptr;