Improve page scrolling performance part 2

Use passive touch gestures as per
https://developers.google.com/web/updates/2017/01/scrolling-intervention

Bug: Issue 8825
Change-Id: If520e287bb5823184909bf12a8c0d31f9c1c2cb7
diff --git a/polygerrit-ui/app/elements/gr-app.html b/polygerrit-ui/app/elements/gr-app.html
index de62646..e4b2577 100644
--- a/polygerrit-ui/app/elements/gr-app.html
+++ b/polygerrit-ui/app/elements/gr-app.html
@@ -19,6 +19,11 @@
   if (localStorage.getItem('USE_SHADOW_DOM') === 'true') {
     window.Polymer = {
       dom: 'shadow',
+      passiveTouchGestures: true,
+    };
+  } else if (!window.Polymer) {
+    window.Polymer = {
+      passiveTouchGestures: true,
     };
   }
 </script>