[Fiber] use srcset to trigger load even on img mount

In facebook#23316 we fixed a bug where onload events were missed if they happened too early. This update adds support for srcset to retrigger the load event. Firefox unfortunately does not trigger a load even when you assign srcset so this won't work in every browser when you use srcset without src however it does close a gap in chrome at least

packages/react-dom-bindings/src/client/ReactFiberConfigDOM.js

@@ -694,6 +694,8 @@ export function commitMount(
     case 'img': {
       if ((newProps: any).src) {
         ((domElement: any): HTMLImageElement).src = (newProps: any).src;
+      } else if ((newProps: any).srcSet) {
+        ((domElement: any): HTMLImageElement).srcset = (newProps: any).srcSet;
       }
       return;
     }