Remove [TreatReturnedNullStringAsNull] from HTMLMediaElement.mediaGroup

Remove [TreatReturnedNullStringAsNull] from HTMLMediaElement.mediaGroup
https://bugs.webkit.org/show_bug.cgi?id=79064

Reviewed by Hajime Morita.

[TreatReturnedNullStringAsNull] is a typo of [TreatReturnedNullStringAs=Null].
But as far as I read the spec
(http://www.whatwg.org/specs/web-apps/current-work/multipage/the-video-element.html#attr-media-mediagroup),
there is no statement about what value should be returned when HTMLMediaElement.mediaGroup
is not yet initialized. In particular, there is no statement that says "null should
be returned when HTMLMediaElement.mediaGroup is not initialized". Thus, instead of
fixing the typo, just removing [TreatReturnedNullStringAsNull] would make sense.
Removing [TreatReturnedNullStringAsNull] does not change the current behavior.

Test: media/media-controller.html (No change in the test results.)

* html/HTMLMediaElement.idl:

git-svn-id: http://svn.webkit.org/repository/webkit/trunk@108305 268f45cc-cd09-0410-ab3c-d52691b4dbfc