Statistics failing with "cannot lock mutex: Invalid argument "

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Several users have complained of getting messages like this:

[Tue Feb 15 10:58:05 2011] [error] [mod_pagespeed 0.9.15.3-404] 
[0215/105805:ERROR:net/instaweb/apache/apr_statistics.cc(141)] 
Variable cache_hits cannot lock mutex: Invalid argument 

This disables the statistics (everything shows up as -1) but should not 
otherwise impact mod_pagespeed functionality.

Original issue reported on code.google.com by abl...@google.com on 17 Feb 2011 at 10:50