Disable overflow menu actions when they are in progress

Adds the ability to disable entries in GR-DROPDOWN via the `disabledIds`
property. GR-CHANGE-ACTIONS uses this to disable overflow menu change
actions in the same way that it disables the buttons for top-level
actions. Tests are added for both elements.

Bug: Issue 5366
Change-Id: Iaeba5f5cb9cfa4d7a89d955a65f3a4134780942a
diff --git a/polygerrit-ui/app/elements/change/gr-change-actions/gr-change-actions_test.html b/polygerrit-ui/app/elements/change/gr-change-actions/gr-change-actions_test.html
index c1b582e..9d27ddc 100644
--- a/polygerrit-ui/app/elements/change/gr-change-actions/gr-change-actions_test.html
+++ b/polygerrit-ui/app/elements/change/gr-change-actions/gr-change-actions_test.html
@@ -399,11 +399,16 @@
     });
 
     test('_setLoadingOnButtonWithKey overflow menu', function() {
-      // TODO(wyatta): Should not throw error when setting loading on an
-      // overflow action. @see Issue 5366
       var key = 'cherrypick';
       var cleanup = element._setLoadingOnButtonWithKey(key);
+      assert.equal(element._actionLoadingMessage, 'Cherry-Picking...');
+      assert.include(element._disabledMenuActions, 'cherrypick');
       assert.isFunction(cleanup);
+
+      cleanup();
+
+      assert.notOk(element._actionLoadingMessage);
+      assert.notInclude(element._disabledMenuActions, 'cherrypick');
     });
 
     suite('revert change', function() {