Bug 1784772 - Forcibly flush scroll position change. r=botond

The lack of the flush is one of the causes of the test timeouts. Without the
flush, the content in question is not overscrollable, thus a swipe gesture
triggers a navigation back to the previous content rather than overscrolling.

Differential Revision: https://phabricator.services.mozilla.com/D172170

widget/tests/browser/browser_test_swipe_gesture.js

@@ -1096,6 +1096,7 @@ add_task(async () => {
   // overscrollable.
   await SpecialPowers.spawn(tab.linkedBrowser, [], async () => {
     content.document.documentElement.scrollLeft = 1;
+    content.document.documentElement.getBoundingClientRect();
     await content.wrappedJSObject.promiseApzFlushedRepaints();
   });