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This CL notifies the accessibility side of the closed multipaste menu.
It helps to update the a11y focus ring's location and generate the
suitable voice announcement.

Bug: 1146473
Change-Id: I3892286f6187a9206426af89c1e442cf78362af4
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2523058
Commit-Queue: Andrew Xu <andrewxu@chromium.org>
Reviewed-by: Alex Newcomer <newcomer@chromium.org>
Cr-Commit-Position: refs/heads/master@{#825162}