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It's not necessary, as a role of "button" with the
"has pop up" state is fully supported and sufficient.
The button_drop_down role wasn't even being mapped
correctly on Windows, leading to inconsistent feedback
with some screen readers.

Bug: 766856

Cq-Include-Trybots: master.tryserver.chromium.linux:closure_compilation
Change-Id: Ib7f631a759ab44ffc3307204bcc74a9471afc22a
Reviewed-on: https://chromium-review.googlesource.com/842475Reviewed-by: David Tseng <dtseng@chromium.org>
Reviewed-by: Avi Drissman <avi@chromium.org>
Reviewed-by: Michael Giuffrida <michaelpg@chromium.org>
Reviewed-by: Scott Violet <sky@chromium.org>
Commit-Queue: Dominic Mazzoni <dmazzoni@chromium.org>
Cr-Commit-Position: refs/heads/master@{#531363}