Cast: Avoid retransmit if we sent the same packet recently (less than RTT)

When transmitting 4Mbit over the "bad" udp_proxy profile, this reduces the
amount of data sent from 12Mbit/s to 7Mbit/s, and seems to make it able to
actually send more frames across the wire and recover faster.

Review URL: https://codereview.chromium.org/343523005

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@278774 0039d316-1c4b-4281-b951-d872f2087c98
24 files changed