Get event targetting working for mouse events.

In 28b50b9614, I committed a patch which broke input handling as I
removed window_manager's aura dependency. This has been fixed for mouse
events, and the wm_flow demo now switches windows on clicks.

Key input is still broken here; that will be handled in another
followup.

BUG=431047
R=sky@chromium.org

Review URL: https://codereview.chromium.org/724973003