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

chrome/browser/prerender/prerender_histograms.h

Index: chrome/browser/prerender/prerender_histograms.h
diff --git a/chrome/browser/prerender/prerender_histograms.h b/chrome/browser/prerender/prerender_histograms.h
index 9e10541f5df71a1bf0d784b22edf5a12daf239fa..61ca823d5a9dd19cc26b94b8bfa6fd9d9b0e43b9 100644
--- a/chrome/browser/prerender/prerender_histograms.h
+++ b/chrome/browser/prerender/prerender_histograms.h
@@ -79,6 +79,10 @@ class PrerenderHistograms {
   // To be called when a new prerender is started.
   void RecordPrerenderStarted(Origin origin) const;
 
+  // To be called when we know how many prerenders are running after starting
+  // a prerender.
+  void RecordConcurrency(size_t prerender_count) const;
+
   // Called when we swap in a prerender.
   void RecordUsedPrerender(Origin origin) const;