In components/net/http_loader.rs::http_fetch we need to set domain_lookup_end (if we're using a persistent connection). Otherwise, domain lookup should be performed by hyper, which is probably out of the scope of this issue.

On getting, the domainLookupEnd attribute MUST return as follows:

1. The same value as fetchStart, if a persistent connection [RFC7230] is used or the resource is retrieved from relevant application caches or local resources.
2. The time immediately after the user agent ends the domain data retrieval from the domain information cache, if the user agent has the domain information in cache.
3. The time immediately before the user agent finishes the domain name lookup for the resource, if the last non-redirected fetch of the resource passes the timing allow check algorithm.
4. zero, otherwise.

Spec: https://w3c.github.io/resource-timing/#dom-performanceresourcetiming-domainlookupend