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This change is partially a cleanup and implementation of UdpTransport
for Cast. As the name suggests it is a UDP transport mechanism for
Cast using net::UdpSocket.

A behavior change is that a Cast receiver can have sender IP address
optional. Only after a packet is received that it can send packets. It
will reply only to that address and ignore packets coming from a
different address.

If a sender address is given to Cast receiver then there is no change.

BUG=331989

Review URL: https://codereview.chromium.org/125713002

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@243854 0039d316-1c4b-4281-b951-d872f2087c98