Add a network request phase for "response waiting to be processed"

cc zoepage digitarald

Compare the two network tracks in this profile: https://perfht.ml/2EejwGq

The network track in the content process looks a lot busier, even though it's listing the same requests. What happens is that we draw the boxes until the event for the completion of the request has been processed on the main thread. The main thread in the content process is a lot busier than the main thread in the parent process, so network request completion waits longer until it's being acknowledged in the main thread, and that results in longer boxes.

For a specific load that shows the difference between the two processes, compare load 53 (timeline.2.1.0.js) in each process's network chart tab.

I think it would be good to make this waiting time explicit in the network chart and in the timing details pane for a request. It's similar to the "Waiting" and "Blocked" phases, but it happens later - it happens after the full response has been received. So I don't know what to call it.

┆Issue is synchronized with this [Jira Task](https://mozilla-hub.atlassian.net/browse/FP-74)