GitLab

AXTreeSourceAura and AXTreeSourceMus share code, so extract a common
base class.

Bug: 851578
Test: views_unittests, views_mus_unittests, unit_tests
Change-Id: I64a245dc6cdb97aaba8e2e05c0a6c7d008a38ef5
Reviewed-on: https://chromium-review.googlesource.com/1117918Reviewed-by: Dominic Mazzoni <dmazzoni@chromium.org>
Reviewed-by: Michael Wasserman <msw@chromium.org>
Commit-Queue: James Cook <jamescook@chromium.org>
Cr-Commit-Position: refs/heads/master@{#571662}