Tweak the public/protected/private sections of MediaControls

After the merge this looked rather strange, so keep as public that
which is used by HTMLMediaElement or MediaControlElements, make
protected that which is used/overridden by MediaControlsAndroid, and
make everything else private.

BUG=341813

Review URL: https://codereview.chromium.org/143703007

git-svn-id: svn://svn.chromium.org/blink/trunk@166911 bbb929c8-8fbe-4397-9dbb-9b2b20218538