Convert NavigationMetricsRecorder to use the new navigation callbacks.

content/public/browser/navigation_handle.h

Index: content/public/browser/navigation_handle.h
diff --git a/content/public/browser/navigation_handle.h b/content/public/browser/navigation_handle.h
index f777975bfd7377e83e9df52216639d5b40ce2de6..6f4afec3e395966b39d15aec2958de5538386f36 100644
--- a/content/public/browser/navigation_handle.h
+++ b/content/public/browser/navigation_handle.h
@@ -177,6 +177,10 @@ class CONTENT_EXPORT NavigationHandle {
   // GetNetErrorCode will be net::OK.
   virtual bool IsErrorPage() = 0;
 
+  // The previous main frame URL that the user was on. This may be empty if
+  // there was no last committed entry.
+  virtual const GURL& GetPreviousURL() = 0;
+
   // Returns the remote address of the socket which fetched this resource.
   virtual net::HostPortPair GetSocketAddress() = 0;