Fix divided in 2 CLs

 1) https://chromium-review.googlesource.com/c/chromium/src/+/1213872
 2) https://chromium-review.googlesource.com/c/chromium/src/+/1213873

Ended up merging the 2 CLs, as I realized that they need to land together (otherwise tests are failing after CL#1).

The following revision refers to this bug:
  https://chromium.googlesource.com/chromium/src.git/+/c77677e8c386d823f08b09f949952af734248b99

commit c77677e8c386d823f08b09f949952af734248b99
Author: dpapad <dpapad@chromium.org>
Date: Tue Sep 11 00:30:39 2018

Fix MaterialBookmarksFocusTest.All test in Polymer 2.

There are 2 separate fixes (but they can't land separately).

1) In Shadow DOM v1 the 'close' events from <dialog> no longer bubbles by default.
   Therefore it needs to be caught and re-fired.

2) The failing test was checking that after closing the action menu, the previously
   focused bookmark-item regains focus. This relied on some intricate timing,
   which was assuming that the browser has finished processing focus by the time the
   'close' listener executes (which is no longer the case after the re-fire of 'close').

Instead, listening explicitly for the 'focus' event on the bookmark-item that is
supposed to be focused fixes the problem.

Bug:  875472 
Change-Id: I74a6061f0967d994a2a5558c3508acbc0652c689
Reviewed-on: https://chromium-review.googlesource.com/1213872
Reviewed-by: Rebekah Potter <rbpotter@chromium.org>
Commit-Queue: Demetrios Papadopoulos <dpapad@chromium.org>
Cr-Commit-Position: refs/heads/master@{#590134}
[modify] https://crrev.com/c77677e8c386d823f08b09f949952af734248b99/chrome/test/data/webui/cr_elements/cr_action_menu_test.js
[modify] https://crrev.com/c77677e8c386d823f08b09f949952af734248b99/chrome/test/data/webui/cr_elements/cr_elements_focus_test.js
[modify] https://crrev.com/c77677e8c386d823f08b09f949952af734248b99/chrome/test/data/webui/md_bookmarks/md_bookmarks_focus_test.js
[modify] https://crrev.com/c77677e8c386d823f08b09f949952af734248b99/testing/buildbot/filters/webui_polymer2_interactive_ui_tests.filter
[modify] https://crrev.com/c77677e8c386d823f08b09f949952af734248b99/ui/webui/resources/cr_elements/cr_action_menu/cr_action_menu.html
[modify] https://crrev.com/c77677e8c386d823f08b09f949952af734248b99/ui/webui/resources/cr_elements/cr_action_menu/cr_action_menu.js