Lo-Fi mode is computing whether to set its custom headers on resource requests using a specific call from the ResourceDispatcherHostDelegate (DetermineEnabledPreviews). With the network service, ResourceDispatcherHost is going away, so this doesn't work anymore.

IIUC Lo-Fi mod correctly, status is determined by page. So, if the main resource is requested with the right header, we could plumb the info at commit time to the renderer process and have the subresources requets made with the right header as well? I think that's more or less what we do, right?

For navigations, could LoFi use 
ContentBrowserClient::NavigationRequestStarted which allows the content embedder to specify custom headers it wants for the navigation? This works for all kinds of navigations (subresources as well). See https://cs.chromium.org/chromium/src/content/public/browser/content_browser_client.h?sq=package:chromium&dr=CSs&l=436.

Ideally, this would also allow to remove most if not all of the plumbing for LoFi inside content/.