GitLab

This CL standardizes the kSelection accessibility event for all the
omnibox popup auxiliary buttons.

This also gives the Header collapse button this accessibility event.

Bug: 1078183, 1052522
Change-Id: Ia56b2e15b47c299bb55f74aa16bb1648b47b322c
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2240298Reviewed-by: Aaron Leventhal <aleventhal@chromium.org>
Commit-Queue: Tommy Li <tommycli@chromium.org>
Cr-Commit-Position: refs/heads/master@{#777555}