fix: add timeToLastByte to onLoad and onContentLoad metrics

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 ?