Remove TreatNullAs=NullString for media interfaces

The spec doesn't have special null handling here:
http://www.whatwg.org/specs/web-apps/current-work/multipage/the-video-element.html

The updated test was run in Firefox Nightly (with media.webvtt.enabled),
IE11 Release Preview and Opera 12.16 (Presto 2.12.388). Everything
passes, with these exceptions:

No other browser supports the mediaGroup property.

The preload default value is "metadata" in IE11 and Presto, and "" in
Firefox. The spec suggests "metadata". Bugs filed:
https://code.google.com/p/chromium/issues/detail?id=310450
https://bugzilla.mozilla.org/show_bug.cgi?id=929890

BUG=310298

Review URL: https://codereview.chromium.org/35803004

git-svn-id: svn://svn.chromium.org/blink/trunk@160318 bbb929c8-8fbe-4397-9dbb-9b2b20218538