Move histograms and supporting code that don't belong in content out.

content/public/browser/web_contents_observer.h

Index: content/public/browser/web_contents_observer.h
===================================================================
--- content/public/browser/web_contents_observer.h	(revision 206723)
+++ content/public/browser/web_contents_observer.h	(working copy)
@@ -259,6 +259,12 @@
   // Invoked before a form repost warning is shown.
   virtual void BeforeFormRepostWarningShow() {}
 
+  // Invoked when the before unload fires. The time is from the renderer.
+  virtual void BeforeUnloadFired(const base::TimeTicks& proceed_time) {}
+
+  // Invoked when a user cancels a before unload dialog.
+  virtual void BeforeUnloadDialogCancelled() {}
+
   // IPC::Listener implementation.
   virtual bool OnMessageReceived(const IPC::Message& message) OVERRIDE;