Let MediaControls use the parent HTMLMediaElement

Likewise, let MediaControlElements use the parent MediaControls.

The goal is that all instances of mediaControllerInterface() should be
replaced by either the media element or its controller. For example,
text tracks and fullscreen will just use the media element, while e.g.
seeking must be done via the MediaController if it is present.

No observable changes are intended in this commit.

BUG=341813

Review URL: https://codereview.chromium.org/195473002

git-svn-id: svn://svn.chromium.org/blink/trunk@169014 bbb929c8-8fbe-4397-9dbb-9b2b20218538