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

chrome/browser/prerender/prerender_manager.h

Index: chrome/browser/prerender/prerender_manager.h
diff --git a/chrome/browser/prerender/prerender_manager.h b/chrome/browser/prerender/prerender_manager.h
index 334157271fa9310d38ebaddb7bd65ab2bf83eea6..edada5a37cda0a8476ef666c82bd88b27e8235b7 100644
--- a/chrome/browser/prerender/prerender_manager.h
+++ b/chrome/browser/prerender/prerender_manager.h
@@ -78,6 +78,7 @@ class PrerenderManager : public base::SupportsWeakPtr<PrerenderManager>,
     PRERENDER_MODE_EXPERIMENT_PRERENDER_GROUP,
     PRERENDER_MODE_EXPERIMENT_5MIN_TTL_GROUP,
     PRERENDER_MODE_EXPERIMENT_NO_USE_GROUP,
+    PRERENDER_MODE_EXPERIMENT_MULTI_PRERENDER_GROUP,
     PRERENDER_MODE_MAX
   };
 
@@ -176,6 +177,7 @@ class PrerenderManager : public base::SupportsWeakPtr<PrerenderManager>,
   static bool ActuallyPrerendering();
   static bool IsControlGroup();
   static bool IsNoUseGroup();
+  static size_t GetMaxConcurrency();
 
   // Query the list of current prerender pages to see if the given web contents
   // is prerendering a page.
@@ -313,6 +315,9 @@ class PrerenderManager : public base::SupportsWeakPtr<PrerenderManager>,
 
   typedef base::hash_map<content::WebContents*, bool> WouldBePrerenderedMap;
 
+  // Time interval before a new prerender is allowed.
+  static const int kMinTimeBetweenPrerendersMs = 500;
+
   // Time window for which we record old navigations, in milliseconds.
   static const int kNavigationRecordWindowMs = 5000;