port r3968 which is supposed to be just a test that a bug isn't present, but in ngx_pagespeed actually triggers that bug.  This needs to be fixed, but is out of scope for a trunk-tracking update.
trunk-tracking: also turn off ProactiveResourceFreshening until we fix PSOL.
Merge pull request #678 from pagespeed/jefftk-update-trunk-tracking

port r3968 which is supposed to be just a test that a bug isn't present...