Drop 'country' from NavTiming graphs; rename to 'browser'

I'd like to start off simple, and splitting the data by country is more than I
can chew at the moment. Also group metrics under 'stats.timers.browser' rather
than 'stats.timers.navigation', for clarity. (I'll prune the old metrics from
Graphite.)

Change-Id: I9f3238adbae2711b7af2328e3fe3b7de6fb4f9f5