[Switch Access] Make Chrome menu openable again

The Chrome menu (the three dots at the right of the omnibar) stopped
being openable recently, because it shows up as a combo box. Modify
the combo box behavior to allow for some elements detected as combo
boxes to not be combo boxes.

This is a temporary fix; the long-term fix is to properly populate
the location for combo box pop-up windows.

AX-Relnotes: n/a.
Bug: 1106529
Change-Id: Ica0494ce85246abb73efb899d15abad38d700ebb
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2300982


Commit-Queue: Anastasia Helfinstein <anastasi@google.com>
Reviewed-by: Dominic Mazzoni <dmazzoni@chromium.org>
Cr-Commit-Position: refs/heads/master@{#791477}