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Previously, this only worked in browser windows. Now, it uses the button
provider to detect if the menu is open, in either browser or hosted app
windows.

Bug: 791399
Change-Id: I885a4979e729eaedbb647903209c54b63286da22
Reviewed-on: https://chromium-review.googlesource.com/991892
Commit-Queue: Matt Giuca <mgiuca@chromium.org>
Reviewed-by: Bret Sepulveda <bsep@chromium.org>
Cr-Commit-Position: refs/heads/master@{#549458}