Start porting the aura FocusController tests to the mojo window manager.

The focus controller tests are set up so that we run (mostly) the same
test code against different ways of accomplishing a focus/activation
change. This takes the first three tests from aura's version of this
file, and tests them against direct api manipulation and mouse events.

BUG=431047
R=sky@chromium.org

Review URL: https://codereview.chromium.org/746703002