NavigationTiming: aggregate data by bits cache host; add 'totalPageLoadTime'

* Aggregate metric data by bits host.
* Adds a 'totalPageLoadTime' metric to the navtiming script and Ganglia view.
  It represents the time from navigationStart to loadEventEnd, or pretty much
  the whole page load. It is not a very actionable metric precisely because it
  is so broad, but it is useful as a gauge of how users are actually
  experiencing the site and where we stand relative to other prominent web
  properties.

Change-Id: I6fbd052b2ae996712fe2b545cdbba1fa01d04288