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This change implements a high priority packet list in PacedSender. In
practice the order of packet priority is:
1. RTCP
2. Audio
3. Video

The reason for doing this such that audio packets can interleave between
video packets even if the audio packets has a greater value of
timestamp. This characteristic of interleaving packets will facilitate
better bandwidth estimation by looking at audio send and ACK pairs.

cast_simulator shows no ill effect but a slight improvement:
Before
Audio frame count received: 17978
Video frame count received: 5169

After
Audio frame count received: 17989
Video frame count received: 5174

BUG=394191

Review URL: https://codereview.chromium.org/399743002

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@283911 0039d316-1c4b-4281-b951-d872f2087c98