Remove a lot of Widget APIs.

We don't need a lot of the Widget API. I also removed the
visibility code which seems to have been broken when we
removed FrameView::show and hide(), which looks like it
would have broken image loading. Unfortuantely we don't
have pixel tests or tests that load images so I can't
test this yet. Even so it's a good simplificatin since
our system has no concept of hidden widgets.

R=abarth@chromium.org, ojan@chromium.org

Review URL: https://codereview.chromium.org/691453002