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Upon duplicated ACKs for a frame the previous behavior was to resend
all packets of the first unacked frame. It was shown in reports that it
is excessive. Instead this change sends the first packet of the last
encoded frame. This significantly reduces the amount of packet
re-transmitted.

PacketStorage is also redesigned in this code. It is now a simple deque
instead of a map.

Review URL: https://codereview.chromium.org/317243007

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@275663 0039d316-1c4b-4281-b951-d872f2087c98