Adds dynamic setting of FEC group size based on the connection's current

Adds dynamic setting of FEC group size based on the connection's current
congestion window.

There will be two triggers for transmitted an FEC packet, when
protection is required -- max_packets_per_fec_group, and an FEC alarm
that triggers an FEC packet if the FEC group has been open for long
enough. This CL implements dynamically setting
max_packets_per_fec_group. The FEC alarm will be written in a
subsequent CL.

Merge internal change: 70821636

R=rch@chromium.org

[Ryan Hamilton, 2014-07-17 00:02:13]

lgtm