Add javascript timing metrics to timing panel onLoad if available

Some browsers support navigation timing via javascript. Since these metrics fit the category of timing, this commit adds a section to the timing panel containing key metrics from the browser, triggered soon after the onLoad event.

The change is plain javascript, so no library dependencies.

Background:
https://dvcs.w3.org/hg/webperf/raw-file/tip/specs/NavigationTiming/Overview.html

Screenshot: 
http://dl.dropbox.com/u/15940111/browser_timing.png

Tested on Chrome 22 and Firefox 14