let parent know about custom width/height

Author: Titanium2099

src/reducers/custom-stage-size.js

@@ -51,6 +51,7 @@ const reducer = function (state, action) {
 };
 
 const setCustomStageSize = function (width, height) {
+    window.parent.postMessage({ type: "block-compiler-action", action: "resizedStage",width: width, height: height}, "*");
     return {
         type: SET_CUSTOM_STAGE_SIZE,
         width,