Calculate cursor stops after change list rendered

Polymer.dom.flush is expensive operation, we can use afterNextRender,
so we don't interrupt polymer rendering. It saves around 100 ms on
dashboard.

Change-Id: I5137397d5893e933f602f335c597136581e33ad2

polygerrit-ui/app/elements/change-list/gr-change-list/gr-change-list.js

@@ -366,9 +366,10 @@
 
     _sectionsChanged() {
       // Flush DOM operations so that the list item elements will be loaded.
-      Polymer.dom.flush();
+      Polymer.RenderStatus.afterNextRender(this, () => {
         this.$.cursor.stops = this._getListItems();
         this.$.cursor.moveToStart();
+      });
     },
 
     _isOutgoing(section) {

polygerrit-ui/app/elements/change-list/gr-change-list/gr-change-list_test.html

@@ -172,11 +172,11 @@ limitations under the License.
         {_number: 2},
       ];
       flushAsynchronousOperations();
+      Polymer.RenderStatus.afterNextRender(element, () => {
         const elementItems = Polymer.dom(element.root).querySelectorAll(
             'gr-change-list-item');
         assert.equal(elementItems.length, 3);
 
-      flush(() => {
         assert.isTrue(elementItems[0].hasAttribute('selected'));
         MockInteractions.pressAndReleaseKeyOn(element, 74, null, 'j');
         assert.equal(element.selectedIndex, 1);
@@ -470,6 +470,7 @@ limitations under the License.
         },
       ];
       flushAsynchronousOperations();
+      Polymer.RenderStatus.afterNextRender(element, () => {
         const elementItems = Polymer.dom(element.root).querySelectorAll(
             'gr-change-list-item');
         assert.equal(elementItems.length, 9);
@@ -507,6 +508,7 @@ limitations under the License.
         assert.equal(change.reviewed, false,
             'Should mark change as unreviewed');
       });
+    });
 
     test('highlight attribute is updated correctly', () => {
       element.changes = [