UMA for measuring how often 2 or more tabs script the same background contents.

content/public/browser/web_contents_observer.h

Index: content/public/browser/web_contents_observer.h
diff --git a/content/public/browser/web_contents_observer.h b/content/public/browser/web_contents_observer.h
index 92b3f530c11d0dce6476314557cf1053c3cd770a..925d5426a4a32e426523273059d5e6959e86288e 100644
--- a/content/public/browser/web_contents_observer.h
+++ b/content/public/browser/web_contents_observer.h
@@ -423,6 +423,9 @@ class CONTENT_EXPORT WebContentsObserver : public IPC::Listener,
   // Notification that |contents| has gained focus.
   virtual void OnWebContentsFocused() {}
 
+  // Notification that opener has changed.
+  virtual void DidChangeOpener() {}
+
   // Notifes that a CompositorFrame was received from the renderer.
   virtual void DidReceiveCompositorFrame() {}