Bug 1518584 - Simplify some comparisons involving the layout viewport. r=kats

Depends on D15973

Differential Revision: https://phabricator.services.mozilla.com/D15974

--HG--
extra : moz-landing-system : lando

gfx/layers/apz/src/AsyncPanZoomController.cpp

@@ -4306,10 +4306,8 @@ void AsyncPanZoomController::NotifyLayersUpdated(
   entertainViewportUpdates = true;
 #endif
   if (entertainViewportUpdates) {
-    if (Metrics().GetLayoutViewport().Width() !=
-            aLayerMetrics.GetLayoutViewport().Width() ||
-        Metrics().GetLayoutViewport().Height() !=
-            aLayerMetrics.GetLayoutViewport().Height()) {
+    if (Metrics().GetLayoutViewport().Size() !=
+            aLayerMetrics.GetLayoutViewport().Size()) {
       needContentRepaint = true;
       viewportUpdated = true;
     }