Re-use logic for opening up download dialog from 'd'

Re-using the logic allows us to focus on the first copy element in
the download dialog when opened from 'd'.

Change-Id: I610f60ed2b556a0b471064d684d8b4b9d9ec8d0b

polygerrit-ui/app/elements/change/gr-change-view/gr-change-view_test.js

diff --git a/polygerrit-ui/app/elements/change/gr-change-view/gr-change-view_test.js b/polygerrit-ui/app/elements/change/gr-change-view/gr-change-view_test.js
index 5624cc7..e0feee9 100644
--- a/polygerrit-ui/app/elements/change/gr-change-view/gr-change-view_test.js
+++ b/polygerrit-ui/app/elements/change/gr-change-view/gr-change-view_test.js
@@ -679,7 +679,9 @@
         });
 
     test('d should open download overlay', () => {
-      const stub = sandbox.stub(element.$.downloadOverlay, 'open');
+      const stub = sandbox.stub(element.$.downloadOverlay, 'open').returns(
+          new Promise(resolve => {})
+      );
       MockInteractions.pressAndReleaseKeyOn(element, 68, null, 'd');
       assert.isTrue(stub.called);
     });