MacViews: Fix app_list_unittests for mouseover change in r311143
Compile of app_list_unittests on toolkit-views enabled Mac regressed in
r311143 because it accessed aura::Window and there is no Aura on Mac
yet.
The MouseHoverToHighlight passes on Mac - it just needs to initialize
the event generator differently.
TapGestureToHighlight is disabled on Mac, since there are no touch
events on desktop Mac.
BUG=441573, 445520, 365977
Review URL: https://codereview.chromium.org/
840373003
Cr-Commit-Position: refs/heads/master@{#311375}