All codec-specific code is hidden within DecoderTraits and available
from generic interfaces. This patch removes the codec-specific
functions from DecoderTraits' public interface.
This change moves the policy of supported media types in the element
nsHTMLMediaElement to the DecoderTraits class. The codec-specific code
has been removed from the media element.
The class DecoderTraits knows about all supported decoders. Adding
an interface to create decoder instances simplifies all users of this
functionality. nsHtmlMediaElement has been updated to use the new
function.
