GitLab

Push some functionality down from AXTreeSourceAura and AXTreeSourceMus
into AXTreeSourceViews.

This allows AXTreeSourceViews to completely implement the AXTreeSource
interface, eliminating the weird "subclasses must implement GetRoot"
condition.

Just cleanup, no functional changes.

Bug: 888145
Test: views_unittests, views_mus_unittests
Change-Id: I1c1b2cfa856fe6f5e5bcf8dfff172a7ffb6e5339
Reviewed-on: https://chromium-review.googlesource.com/c/1351776Reviewed-by: Dominic Mazzoni <dmazzoni@chromium.org>
Commit-Queue: James Cook <jamescook@chromium.org>
Cr-Commit-Position: refs/heads/master@{#611112}