Recently I landed a few changes on ViewEventTestBase for the creation of aura::Env (crrev.com/592563) and the creation of UIControls (crrev.com/592252).

Then, looking into other failures based on ViewEventTestBase, I've come to realize that ViewEventTestBase does not use Mus even when Mash or SingleProcessMash is enabled.

It'll use ash::Shell::GetPrimaryRootWindow() for the creation of windows (https://chromium.googlesource.com/chromium/src/+/5b4591be80b1a49dfceceda6b4ef28b5ab68af64/chrome/test/base/view_event_test_platform_part_chromeos.cc#41) thus the testing window (and its view) is managed within Ash.

It is still not clear to me if this is the real cause of the failures -- but this will be confusing. Maybe we should introduce initialization of MusClient as views_mus_interactive_ui_tests does, or revert the Env/UIControls change so that the view resides within Ash (and be tested there).