Looking at it again, I looks like the rewritedriverfactory takes ownership here of the `fetcher` that gets returned:
https://code.google.com/p/modpagespeed/source/browse/trunk/src/net/instaweb/rewriter/rewrite_driver_factory.cc#514
And deletes it here:
https://code.google.com/p/modpagespeed/source/browse/trunk/src/net/instaweb/rewriter/rewrite_driver_factory.cc#137