Fix gr-diff-comment-test on Firefox

The keydown event doesn’t fire when when the target of the
mock interaction is the inner textarea instead of the custom
element on firefox.

Change-Id: I630babca72ff2e28bab03540b34f2d66be912c66

polygerrit-ui/app/test/gr-diff-comment-test.html

@@ -196,8 +196,7 @@ limitations under the License.
       });
       MockInteractions.tap(element.$$('.cancel'));
       MockInteractions.tap(element.$$('.discard'));
-      MockInteractions.pressAndReleaseKeyOn(
-          element.$.editTextarea.textarea, 27);  // 'esc'
+      MockInteractions.pressAndReleaseKeyOn(element.$.editTextarea, 27); // esc
     });
 
     test('draft saving/editing', function(done) {