Make popup buttons inherit setsize from controlled element.

This change:
1. Adds logic for popup buttons to inherit setsize from a controlled
element. This is important because ChromeVox reports setsize when
focusing a popup button and we want to make sure it reports the correct
value.
2. Adds an AXTreeTest for new logic.
3. Adds ChromeVox test for new logic.
4. Splits flakey test, which was disabled, into two separate tests
which both pass.

expanded popup buttons.

Bug: 1108895
Change-Id: I45e70c8ec20123e0194159f9e884daf4413b6c23
AX-Relnotes: Fix issue where ChromeVox was reporting "0 items" for
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2315352


Commit-Queue: Akihiro Ota <akihiroota@chromium.org>
Reviewed-by: Dominic Mazzoni <dmazzoni@chromium.org>
Cr-Commit-Position: refs/heads/master@{#791464}