Display Alternate Protocol Probability Threshold in net-internals.

Also, add a comment to SetAlternateProtocolProbabilityThreshold.

Review URL: https://codereview.chromium.org/577103002

Cr-Commit-Position: refs/heads/master@{#295471}
diff --git a/net/http/http_network_session.cc b/net/http/http_network_session.cc
index 9c2cc14..6dd06ea2 100644
--- a/net/http/http_network_session.cc
+++ b/net/http/http_network_session.cc
@@ -268,6 +268,8 @@
                    params_.enable_quic_time_based_loss_detection);
   dict->SetString("origin_to_force_quic_on",
                   params_.origin_to_force_quic_on.ToString());
+  dict->SetDouble("alternate_protocol_probability_threshold",
+                  params_.alternate_protocol_probability_threshold);
   return dict;
 }