Use about:blank as the failback URL if the filter denies a navigation.

content/browser/renderer_host/render_view_host_impl.h

Index: content/browser/renderer_host/render_view_host_impl.h
===================================================================
--- content/browser/renderer_host/render_view_host_impl.h	(revision 127913)
+++ content/browser/renderer_host/render_view_host_impl.h	(working copy)
@@ -390,10 +390,12 @@
     return is_waiting_for_unload_ack_;
   }
 
-  // Checks that the given renderer can request |url|, if not it sets it to an
-  // empty url.
+  // Checks that the given renderer can request |url|, if not it sets it to
+  // about:blank.
+  // empty_allowed must be set to false for navigations for security reasons.
   static void FilterURL(ChildProcessSecurityPolicyImpl* policy,
                         int renderer_id,
+                        bool empty_allowed,
                         GURL* url);
 
   // NOTE: Do not add functions that just send an IPC message that are called in