first 3 histograms under /mod_pagespeed_statistics are empty

[GoogleCodeExporter]

This can be seen on mod_pagespeed.com/mod_pagespeed_statistics.

Pagespeed Resource Latency Histogram
Rewrite Latency Histogram
Backend Fetch First Byte Latency Histogram

All 3 of those have no data.  The HTML latency histogram has data.  We should 
either populate the other 3 histograms or remove them from the display.

Original issue reported on code.google.com by jmara...@google.com on 3 Apr 2012 at 2:52