Clean up MediaControlElement virtual and unused functions

This class was a bit confusing:

isShowing() was never used.

The state of MediaControlElement::shadowPseudoId() was odd. The same
virtual function exists on Element, and the MediaControl*Element
classes inherit both Element and MediaControlElement. When they
override shadowPseudoId() they override both of their base classes.
However, MediaControlElement::shadowPseudoId() wasn't ever used...

The remaining functions don't need to be virtual since they're never
overridden.

BUG=none

Review URL: https://codereview.chromium.org/145503008

git-svn-id: svn://svn.chromium.org/blink/trunk@166767 bbb929c8-8fbe-4397-9dbb-9b2b20218538