Adds a second vector of streams to FFmpegDemuxer to handle re-mapped audio/video streams.

      For media containing non-audio/video streams we were properly exposing the correct streams via Demuxer::GetStream() (i.e., no data streams were exposed) however we weren't properly re-routing the packets to the correct streams during demuxing.

Before:
  Media contains [DATA, AUDIO, VIDEO]
  We create supported vector [AUDIO, VIDEO]
  When demuxing a packet who says they are from stream 1 (AUDIO), we would incorrectly assign it to VIDEO

After:
  Media contains [DATA, AUDIO, VIDEO]
  We create supported vector [AUDIO, VIDEO]
  We create extra packet vector [NULL, AUDIO, VIDEO]
  When demuxing, we now correctly assign it to the proper stream and automatically free unused packets

Review URL: http://codereview.chromium.org/100085

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@14742 0039d316-1c4b-4281-b951-d872f2087c98