Bug 1578008 - Make promiseRDMZoom more simple and reliable. r=bradwerth

Now that we no longer have the various resolution shenanigans, we really
just want to ensure that child processes observe the resize of the
<browser> element, which can happen just fine if we ensure to flush
layout.

Differential Revision: https://phabricator.services.mozilla.com/D72552

devtools/client/responsive/test/browser/browser.ini

@@ -93,4 +93,3 @@ tags = devtools geolocation
 [browser_viewport_zoom_toggle.js]
 [browser_window_close.js]
 [browser_window_sizing.js]
-skip-if = (os == 'linux' && bits == 64) || (os == 'mac') #Bug 1578008

devtools/client/responsive/test/browser/head.js

@@ -924,22 +924,22 @@ function promiseContentReflow(ui) {
 // This function returns a promise that will be resolved when the
 // RDM zoom has been set and the content has finished rescaling
 // to the new size.
-function promiseRDMZoom(ui, browser, zoom) {
-  return new Promise(resolve => {
+async function promiseRDMZoom(ui, browser, zoom) {
   const currentZoom = ZoomManager.getZoomForBrowser(browser);
   if (currentZoom.toFixed(2) == zoom.toFixed(2)) {
-      resolve();
     return;
   }
 
-    const zoomComplete = BrowserTestUtils.waitForEvent(
-      browser,
-      "FullZoomResolutionStable"
-    );
+  const width = browser.getBoundingClientRect().width;
+
   ZoomManager.setZoomForBrowser(browser, zoom);
 
-    // Await the zoom complete event, then reflow.
-    zoomComplete.then(promiseContentReflow(ui)).then(resolve);
+  // RDM resizes the browser as a result of a zoom change, so we wait for that.
+  //
+  // This also has the side effect of updating layout which ensures that any
+  // remote frame dimension update message gets there in time.
+  await BrowserTestUtils.waitForCondition(function() {
+    return browser.getBoundingClientRect().width != width;
   });
 }