Only commit cookie changes in prerenders after a prerender is shown

content/public/browser/content_browser_client.h

Index: content/public/browser/content_browser_client.h
===================================================================
--- content/public/browser/content_browser_client.h	(revision 269781)
+++ content/public/browser/content_browser_client.h	(working copy)
@@ -234,6 +234,10 @@
   virtual bool IsSuitableHost(RenderProcessHost* process_host,
                               const GURL& site_url);
 
+  // Returns whether a new view for a new site instance can be added to a
+  // given |process_host|.
+  virtual bool MayReuseHost(RenderProcessHost* process_host);
+
   // Returns whether a new process should be created or an existing one should
   // be reused based on the URL we want to load. This should return false,
   // unless there is a good reason otherwise.
@@ -626,7 +630,7 @@
   // if the default cookie store should be used
   // This is called on the IO thread.
   virtual net::CookieStore* OverrideCookieStoreForRenderProcess(
-      int render_process_id_);
+      int render_process_id);
 
 #if defined(VIDEO_HOLE)
   // Allows an embedder to provide its own ExternalVideoSurfaceContainer