Remove what remains of MediaControllerInterface

The currentTime() and duration() should simply use the media element, as
that is what paintMediaSlider() in RenderMediaControls uses, so the
existing code would not work if the media element and its controller had
different durations.

setCurrentTime() does require taking the controller into account, so add
some tests and expand that final instance of mediaControllerInterface().

BUG=341813, 341852
TEST=LayoutTests/media/controls-timeline.html
     LayoutTests/media/controls-timeline-with-controller.html

Review URL: https://codereview.chromium.org/219283004

git-svn-id: svn://svn.chromium.org/blink/trunk@170512 bbb929c8-8fbe-4397-9dbb-9b2b20218538