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 29fbf5f73487d0d85706384aa14609f90e81a726..5cace03bc25256e2abb4e7560c24ddaf3b76f726 100644
--- a/chrome/browser/io_thread.h
+++ b/chrome/browser/io_thread.h
@@ -172,6 +172,7 @@ 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;
@@ -358,6 +359,12 @@ 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_;