Use SpecialWrapFor over HTMLMediaElement.

To appropriately delegate the creation of a wrapper to
subclasses of HTMLMediaElement, annotate HTMLMediaElement's
interface with SpecialWrapFor.

R=haraken@chromium.org
BUG=379499

Review URL: https://codereview.chromium.org/483863003

git-svn-id: svn://svn.chromium.org/blink/trunk@180555 bbb929c8-8fbe-4397-9dbb-9b2b20218538