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 - Emit 'open-changed' events from cr-action-menu and add test.
 - Update viewer-pdf-toolbar-new to leverage these.
 - Change viewer-download-controls to use same approach.
   Previously relied on manually updating a boolean when
   the menu was triggered and when it was closed.

Fixed: 1130107
Change-Id: I9303cd4a6cc4befd5aaf99e7eff9b0b95a8ec5d0
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2421184Reviewed-by: John Lee <johntlee@chromium.org>
Commit-Queue: dpapad <dpapad@chromium.org>
Cr-Commit-Position: refs/heads/master@{#808914}