[Multipaste] Speak a11y announcement when delete button is focused

This CL does two things:
(1) Ignore the MenuItemView instance in accessibility events handling.
Then we can customize accessibility behaviors of the multipaste menu
in ClipboardHistoryItemView class.

(2) When the delete button gets the pseudo focus, the accessibility
side is notified.

Bug: 1145773
Change-Id: Ibd73b04637b89cc8200ca2f74721a10a44e857c4
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2513502


Reviewed-by: Alex Newcomer <newcomer@chromium.org>
Commit-Queue: Andrew Xu <andrewxu@chromium.org>
Cr-Commit-Position: refs/heads/master@{#824561}