Remove aura and make a pure mojo::View version of the aura::Window FocusController.

This patch is only half done, however the main driver (removing aura) is needed by other teams for them to make progress.

Input handling is regressed in this patch; while FocusController is rebuilt to the point where it should theoretically work, all of the details about ui::EventTargeter don't work correctly. There is an empty subclass ViewTargeter which will need to be filled out before

Thankfully, this should only regress people who are using the wm_flow demo, and this should unblock the android guys who don't have aura support on their platform.

BUG=431047
R=eseidel@chromium.org, sky@chromium.org

Review URL: https://codereview.chromium.org/698543005