Use the media controls play button internal state for painting

MediaControlElement::displayType() is not actually used for rendering,
the logic is instead duplicated in RenderMediaControls, sometimes with
minor differences. When scrubbing was made internal to the controls in
r169814, I neglected to update the corresponding rendering code.

BUG=359522

Review URL: https://codereview.chromium.org/239113003

git-svn-id: svn://svn.chromium.org/blink/trunk@171706 bbb929c8-8fbe-4397-9dbb-9b2b20218538