fix: add timeToLastByte to onLoad and onContentLoad metrics

[william-p]

From the spec:

* onContentLoad [number, optional] - Content of the page loaded. Number of milliseconds since page load started (page.startedDateTime). Use -1 if the timing does not apply to the current request.
* onLoad [number,optional] - Page is loaded (onLoad event fired). Number of milliseconds since page load started (page.startedDateTime). Use -1 if the timing does not apply to the current request.

From Phantomas:
Times (windowOnLoadTime, onDOMReadyTime) are relative to responseEnd entry in NavigationTiming (represented by timeToLastByte metric)

I've added timeToLastByte to windowOnLoadTime and onDOMReadyTime metrics.
I'm not sure if my fix is correct, what do you think ?

[macbre]

Let's follow the spec :)