Update NavigationTiming StatsD reporter to schema rev. 5832704

This patch updates navtiming.py, the script that reads Navigation Timing data
and reports it to StatsD. It makes the script homogenize events logged under
the older & newer schemas, to ensure the Ganglia graphs continue to be updated.
Some time after the deployment of rev. 5832704, I'll remove the logic that
homogenizes both event schemas into a common format.

Also updates the Ganglia view to include a new 'pageSpeed' metric, representing
the time from domInteractive to loadEventEnd.

Change-Id: I3027fbdb76a04350ac61bc5e90d06bed0d6bb2a9
Merge "Update NavigationTiming StatsD reporter to schema rev. 5832704" into production