Refactor RTCVideoDecoderFactoryTv

This is to follow a change in media::Demuxer in
https://codereview.chromium.org/26492004

BUG=

Review URL: https://codereview.chromium.org/26267004

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@228419 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 228419