Move Widget::setCursor to FrameView

Nothing calls this through a Widget*, they always call it through a
FrameView*. This also lets us devirtualize setCursor since it doesn't
need to be on HostWindow anymore. In cleaning this up I also realized
that Widget::hostWindow() had a line of noop code and then an
ASSERT_NOT_REACHED() which I removed.

Review URL: https://codereview.chromium.org/32633005

git-svn-id: svn://svn.chromium.org/blink/trunk@160225 bbb929c8-8fbe-4397-9dbb-9b2b20218538