Make three simultanious prerenders the default maximum in Canary and Dev only.

chrome/browser/prerender/prerender_config.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_PRERENDER_PRERENDER_CONFIG_H_
 #define CHROME_BROWSER_PRERENDER_PRERENDER_CONFIG_H_
 
+#include <string>
+
 #include "base/time.h"
 #include "ui/gfx/rect.h"
 
 namespace prerender {
 
 struct Config {
   Config();
 
   // Maximum memory use for a prerendered page until it is killed.
   size_t max_bytes;
 
   // Number of simultaneous prendered pages allowed.
-  unsigned int max_elements;
+  size_t max_concurrency;
 
   // Is rate limiting enabled?
   bool rate_limit_enabled;
 
   // Maximum age for a prerendered page until it is removed.
   base::TimeDelta max_age;
 
   // Is https allowed?
   bool https_allowed;
 
   // The default tab bounds used as the prerenderer tab size when the active tab
   // cannot be accessed.
   gfx::Rect default_tab_bounds;
 
   // User agent being used as an override for the WebContents.
   std::string user_agent_override;
 
   // Is the user agent being overridden for this NavigationEntry?
   bool is_overriding_user_agent;
 };
 
 }  // namespace prerender
 
 #endif  // CHROME_BROWSER_PRERENDER_PRERENDER_CONFIG_H_
+