Revert of Remove support for Alt-Svc/Alternate Protocol Probability

ios/chrome/browser/ios_chrome_io_thread.h

Index: ios/chrome/browser/ios_chrome_io_thread.h
diff --git a/ios/chrome/browser/ios_chrome_io_thread.h b/ios/chrome/browser/ios_chrome_io_thread.h
index 58017133956be905543b191b5ce4a72acacc48bf..67faf2d0556271fd4b91b52ab577669e2a813648 100644
--- a/ios/chrome/browser/ios_chrome_io_thread.h
+++ b/ios/chrome/browser/ios_chrome_io_thread.h
@@ -138,6 +138,7 @@
     Optional<bool> enable_http2;
     Optional<bool> parse_alternative_services;
     Optional<bool> enable_alternative_service_with_different_host;
+    Optional<double> alternative_service_probability_threshold;
 
     Optional<bool> enable_npn;
 
@@ -349,6 +350,11 @@
   static net::QuicTagVector GetQuicConnectionOptions(
       const VariationParameters& quic_trial_params);
 
+  // Returns the alternative service probability threshold specified by
+  // any flags in |quic_trial_params|.
+  static double GetAlternativeProtocolProbabilityThreshold(
+      const VariationParameters& quic_trial_params);
+
   static net::URLRequestContext* ConstructSystemRequestContext(
       Globals* globals,
       net::NetLog* net_log);