Add code to keep track of what fraction of pageviews are top sites.

chrome/browser/prerender/prerender_observer.h

Index: chrome/browser/prerender/prerender_observer.h
===================================================================
--- chrome/browser/prerender/prerender_observer.h	(revision 98138)
+++ chrome/browser/prerender/prerender_observer.h	(working copy)
@@ -46,7 +46,7 @@
   void OnMsgUpdateTargetURL(int32 page_id, const GURL& url);
 
   // Retrieves the PrerenderManager, or NULL, if none was found.
-  PrerenderManager* MaybeGetPrerenderManager();
+  PrerenderManager* MaybeGetPrerenderManager() const;
 
   // Checks with the PrerenderManager if the specified URL has been preloaded,
   // and if so, swap the RenderViewHost with the preload into this TabContents
@@ -61,6 +61,8 @@
   // Also resets the hover to no hover.
   void MaybeLogCurrentHover(bool was_used);
 
+  bool IsTopSite(const GURL& url) const;
+
   // TabContentsWrapper we're created for.
   TabContentsWrapper* tab_;