The current PageLoadTracker/PageLoadMetricsObserver implementation is a little confusing when it comes to tracking of non-HTML resources.

The render process doesn't report any timing data for these resources, but we do instantiate PageLoadMetricsObservers and invoke OnComplete on them for non-HTML main resouces.

For most observers, then ends up being ok, because observers often check to see if the PageLoadTiming passed to OnComplete is empty before logging metrics.

However, for observers that don't care about timing, such as the https engagement observer, it's non-obvious that you need to check that timing is empty to filter out non-HTML main resource loads.

apf indicated that she doesn't want to include non-HTML main resource loads in the https engagement observer.

For now, we should add a check for timing.IsEmpty to fix the broken behavior.

The right longer term fix is to stop calling OnComplete for non-HTML observers, as all of our existing observers only want to track HTML main resources. If in the future we have observers that want to track non-HTML resources, we'll need to add a mechanism to allow them to opt in to tracking other resource types, since is the non-default behavior.