Move ReadyState enum from MediaControllerInterface to HTMLMediaElement

The spec for MediaController.readyState says "uses
HTMLMediaElement.readyState's values", so make it so in the
implementation as well.

This is a first step in reducing the size of MediaControllerInterface.

BUG=341852

Review URL: https://codereview.chromium.org/154283004

git-svn-id: svn://svn.chromium.org/blink/trunk@166764 bbb929c8-8fbe-4397-9dbb-9b2b20218538