relnote: QUIC bugfix (and test) for setting smaller receive window size

relnote:  QUIC bugfix (and test) for setting smaller receive window size
under flow-control auto-tuning.   Behavior gated by finch tag AFCW,
which is currently enabled in canary and dev.

Prior to this fix, streams established before the crypto handshake,
i.e. headers and crypto, would have their flow control window size
become out of sync between client and server, when tags to set
smaller initial window sizes were used (kIFW*).   It is believed
this was causing some flow control deadlocks in the auto-tuning trial.
This change also corrects an oversight, adding logic so that kIFW*
reduce the initial session flow control window size as well.

Merge internal change: 101695854

R=rch@chromium.org

[Ryan Hamilton, 2015-09-10 14:28:49]

lgtm