[SPDY] Do not advertise SPDY/2 by default

chrome/browser/io_thread.h

 // Copyright (c) 2012 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 #ifndef CHROME_BROWSER_IO_THREAD_H_
 #define CHROME_BROWSER_IO_THREAD_H_
 
 #include <string>
 
 #include "base/basictypes.h"
@@ skipping 203 matching lines @@
 
   // BrowserThreadDelegate implementation, runs on the IO thread.
   // This handles initialization and destruction of state that must
   // live on the IO thread.
   virtual void Init() OVERRIDE;
   virtual void InitAsync() OVERRIDE;
   virtual void CleanUp() OVERRIDE;
 
   void InitializeNetworkOptions(const CommandLine& parsed_command_line);
 
-  // Enable the SPDY protocol.  If this function is not called, SPDY/3
-  // will be enabled.
+  // Enable SPDY with the given mode, which may contain the following:
+  //
   //   "off"                      : Disables SPDY support entirely.
   //   "ssl"                      : Forces SPDY for all HTTPS requests.
   //   "no-ssl"                   : Forces SPDY for all HTTP requests.
   //   "no-ping"                  : Disables SPDY ping connection testing.
   //   "exclude=<host>"           : Disables SPDY support for the host <host>.
   //   "no-compress"              : Disables SPDY header compression.
   //   "no-alt-protocols          : Disables alternate protocol support.
-  //   "force-alt-protocols       : Forces an alternate protocol of SPDY/2
+  //   "force-alt-protocols       : Forces an alternate protocol of SPDY/3
   //                                on port 443.
   //   "single-domain"            : Forces all spdy traffic to a single domain.
   //   "init-max-streams=<limit>" : Specifies the maximum number of concurrent
   //                                streams for a SPDY session, unless the
   //                                specifies a different value via SETTINGS.
   void EnableSpdy(const std::string& mode);
 
   // Global state must be initialized on the IO thread, then this
   // method must be invoked on the UI thread.
   void InitSystemRequestContext();
@@ skipping 74 matching lines @@
 
   // True if SPDY is disabled by policy.
   bool is_spdy_disabled_by_policy_;
 
   base::WeakPtrFactory<IOThread> weak_factory_;
 
   DISALLOW_COPY_AND_ASSIGN(IOThread);
 };
 
 #endif  // CHROME_BROWSER_IO_THREAD_H_