Preparation for Chrome experiment to enable bandwidth resumption.

Preparation for Chrome experiment to enable bandwidth resumption.
Protected behind FLAGS_quic_enable_bandwidth_resumption_experiment

Once this CL is in, I intend to add a Chrome experiment to add the
connection option kBWRE to a small percentage of connections to test the
effect of turning on BW resumption.

If a client provides a previously cached bandwidth estimate (that is
recent, and from the same serving region), then set initial CWND based
on this data.

Behavior change only when both the flag is true, and the Chrome
experiment adds the connection option to client handshakes.

A followup CL (78446219) will add varz to track the distribution of
resulting initial CWNDs.

Merge internal change: 80239310

R=rch@chromium.org

[Ryan Hamilton, 2014-11-20 01:07:07]

lgtm