Allow IME on focus() after first user gesture since load.

This is (hopefully) the final patch in our experiment to update the focus
heuristic. It might cause issues with sites that focus on some events (e.g.
onresize) causing the osk to constantly pop up. In that case we can revert and
tweak this some more.

I noticed that UserGestureNotifier was basically trying to track if there had
been a gesture since load and then notify the autofill client so I've updated
its semantics and shared the implementation.

BUG=371927

Review URL: https://codereview.chromium.org/394223004

git-svn-id: svn://svn.chromium.org/blink/trunk@178708 bbb929c8-8fbe-4397-9dbb-9b2b20218538