Add a probability to Alternate-Protocol support.

chrome/browser/io_thread.h

Index: chrome/browser/io_thread.h
diff --git a/chrome/browser/io_thread.h b/chrome/browser/io_thread.h
index ca817394e8bc1f70737a68e7f1c24e5a117e1f9b..919f3741f4da72858fba22aa356a63bc2ea46537 100644
--- a/chrome/browser/io_thread.h
+++ b/chrome/browser/io_thread.h
@@ -173,6 +173,7 @@ class IOThread : public content::BrowserThreadDelegate {
     Optional<bool> force_spdy_always;
     std::set<net::HostPortPair> forced_spdy_exclusions;
     Optional<bool> use_alternate_protocols;
+    Optional<double> alternate_protocol_probability_threshold;
     Optional<bool> enable_websocket_over_spdy;
 
     Optional<bool> enable_quic;
@@ -361,6 +362,12 @@ class IOThread : public content::BrowserThreadDelegate {
   static net::QuicTagVector ParseQuicConnectionOptions(
       const std::string& connection_options);
 
+  // Returns the alternate protocol probability threshold specified by
+  // any flags in |command_line| or |quic_trial_params|.
+  static double GetAlternateProtocolProbabilityThreshold(
+      const base::CommandLine& command_line,
+      const VariationParameters& quic_trial_params);
+
   // The NetLog is owned by the browser process, to allow logging from other
   // threads during shutdown, but is used most frequently on the IOThread.
   ChromeNetLog* net_log_;