[WebM] Consider generating 'cenc:default_KID' attribute in ContentProtection element

[kqyang]

This is what the ContentProtection element looks like in mpd for webm mpd:

<ContentProtection schemeIdUri="urn:uuid:edef8ba9-79d6-4ace-a3c8-27dcd51d21ed">
<cenc:pssh>
AAAAT3Bzc2gAAAAA7e+LqXnWSs6jyCfc1R0h7QAAAC8IARIQGRgWXzFoWs60ER762nZScRoNd2lkZXZpbmVfdGVzdCIIMDEyMzQ1NjcyAA==
</cenc:pssh>
</ContentProtection>

Compared to ContentProtection elements for mp4 mpd:

<ContentProtection value="cenc" schemeIdUri="urn:mpeg:dash:mp4protection:2011"
                   cenc:default_KID="31323334-3536-3738-3930-313233343536"/>
<ContentProtection schemeIdUri="urn:uuid:edef8ba9-79d6-4ace-a3c8-27dcd51d21ed">     
<cenc:pssh>
AAAAMHBzc2gAAAAA7e+LqXnWSs6jyCfc1R0h7QAAABAxMjM0NTY3ODkwMTIzNDU2
</cenc:pssh>
</ContentProtection>

cenc:default_KID is useful to indicate that this content is encrypted using the key associated with this key id. Some players use cenc:default_KID to match DASH Representations to key statuses. We should consider generating cenc:default_KID in webm mpd too.

It could be something like this:

<ContentProtection schemeIdUri="urn:uuid:edef8ba9-79d6-4ace-a3c8-27dcd51d21ed" 
                   cenc:default_KID="31323334-3536-3738-3930-313233343536">
<cenc:pssh>
AAAAT3Bzc2gAAAAA7e+LqXnWSs6jyCfc1R0h7QAAAC8IARIQGRgWXzFoWs60ER762nZScRoNd2lkZXZpbmVfdGVzdCIIMDEyMzQ1NjcyAA==
</cenc:pssh>
</ContentProtection>

[kqyang]

@TheModMaker