Fix more issues with gr-dropdown

In troubleshooting this element for shadow DOM, some more issues arose.

Change-Id: I3843d6364835e7fd170dc824ce68fa722a701759

polygerrit-ui/app/elements/shared/gr-dropdown/gr-dropdown.html

@@ -167,7 +167,7 @@ limitations under the License.
         cursor-target-class="selected"
         scroll-behavior="never"
         focus-on-move
-        stops="[[_listElements]]"</gr-cursor-manager>
+        stops="[[_listElements]]"></gr-cursor-manager>
     <gr-rest-api-interface id="restAPI"></gr-rest-api-interface>
   </template>
   <script src="gr-dropdown.js"></script>

polygerrit-ui/app/elements/shared/gr-dropdown/gr-dropdown.js

@@ -196,7 +196,8 @@
 
     _resetCursorStops() {
       Polymer.dom.flush();
-      this._els = this.querySelectorAll('li');
+      // TODO(kaspern): This is broken in shadow DOM.
+      this._listElements = this.querySelectorAll('li');
     },
   });
 })();