The page load metrics code as 2 code paths:
- the non network service one, that relies on the ResourceDispatcherHost (see chrome_resource_dispatcher_host_delegate.cc NotifyUIThreadOfRequestComplete)
- the network service one, that used the WebContentsObserver notifications (see MetricsWebContentsObserver::ResourceLoadComplete)


We can't switch to the network service one yet as we still need the data reduction proxy data which is not available yet with the network service.

Once it is available, we should hook it in in MetricsWebContentsObserver::ResourceLoadComplete and removed the non network service code path.