trunk-tracking: update to r3193

* statistics_logging_file_prefix is gone, replaced with log_dir
* propagate a units fix from apache_system_test
* test nonces for critical css beaconing

Previous trunk-tracking was at 3162.  Diff of apache_system_test.sh:
  https://code.google.com/p/modpagespeed/source/diff?path=/trunk/src/install/apache_system_test.sh&format=side&r=3193&old_path=/trunk/src/install/apache_system_test.sh&old=3162
Merge pull request #414 from pagespeed/jefftk-update-trunk-tracking

trunk-tracking: update to r3193