Add measurement from nav start to just before 'onload' event

This is apparently the base figure that GA computes for 'page load time'. It is
not very granular and thus less helpful than some of the other measurements,
but until we ramp up our capacity to analyze the data we can use this figure as
an overall indicator of user-perceived latency.

Change-Id: Ide5f832e1e88ef998ad010efb77c4360c0333492
Merge "Add measurement from nav start to just before 'onload' event"