Added helper methods to get/set previous_cached_network_params_.

Added helper methods to get/set previous_cached_network_params_.

Merging the following internal change:

Add varz to track accuracy of QUIC bandwidth predictions.

Plan is to use this data to calculate possible impact of using bandwidth
predictions, get an idea of how accurate our predictions are. Intended
varz output is something like:

.../spdy4_over_quic/stk_predicted_bw_accuracy_within_5_mins
.../spdy4_over_quic/stk_predicted_bw_accuracy_within_50_mins
.../spdy4_over_quic/stk_predicted_bw_accuracy_within_500_mins
.../spdy4_over_quic/stk_predicted_bw_accuracy_any_time
.../spdy4_over_quic/max_stk_predicted_bw_accuracy_within_5_mins
.../spdy4_over_quic/max_stk_predicted_bw_accuracy_within_50_mins
.../spdy4_over_quic/max_stk_predicted_bw_accuracy_within_500_mins
.../spdy4_over_quic/max_stk_predicted_bw_accuracy_any_time

Each of these is a map of 20 buckets, covering 10% chunks of the
[0%, 200%] range. For example, a bandwidth estimate from 25 minutes ago,
that turns out to be 125% of the observed bandwidth, will result in the
stk_predicted_bw_accuracy_within_50_mins_histogram{13} bucket being
incremented.

Merge internal change: 77877600

R=rch@chromium.org

[Ryan Hamilton, 2014-10-20 21:23:40]

lgtm

Might want to clean up the CL description since it includes some internal
details. Nothing important, though.