Fix double-parenting in AXWindowObjWrapper under mash

Under mus/mash both the WindowTreeHost's root aura::Window and the content
aura::Window will return the same Widget for GetWidgetForNativeView(). Only
return the Widget for the content window, not the root, since otherwise
we'll end up with two children in the AX node tree that have the same
parent.

This allows AXAuraWindowUtilsTest to run under views_mus_unittests and
unblocks some future tests.

Bug: 911945, 911776
Test: views_unittests, views_mus_unittests
Change-Id: I554c77e0dca90f1901a42d3331bca0a05e7bbd88
Reviewed-on: https://chromium-review.googlesource.com/c/1365986
Reviewed-by: Dominic Mazzoni <dmazzoni@chromium.org>
Commit-Queue: James Cook <jamescook@chromium.org>
Cr-Commit-Position: refs/heads/master@{#614515}
2 files changed