Do not warn on "Invalid url relative to..."

[GoogleCodeExporter]

This happens whenever a resource is loaded from another domain that the site 
owner has not authorized for rewriting.  The warnings clutter the logs and 
possibly reduce performance due to disk pressure.

We can, however, increment a statistic so we know the weight of the resources 
served from external sources.

Original issue reported on code.google.com by jmara...@google.com on 6 Nov 2010 at 10:26