Servo: Websocket related tasks should be using a dedicated task source `websocket-task-source`

spec: https://html.spec.whatwg.org/multipage/web-sockets.html#websocket-task-source

@AgustinCB Sorry I might have pointed you in the wrong direction with https://github.com/servo/servo/issues/21589, see https://github.com/servo/servo/pull/21609#issuecomment-418579585

However I think this one is likely to be less "controversial" :)

In implementing this task-source, it would be useful to look at all the places in the websocket part of the spec referencing "queue a task" and ensuring the new task-source is used in the corresponding places in the Servo codebase.

Note that in some cases, a specific task in a section, like the websocket section, could still end-up using the networking, or another, task source. I didn't see this in the websocket section on an initial read, but I know for example this to be the case in the event-source section(where for example the "processing model" refers to the networking task-source, whereas the rest of the section is also described as "The task source for any tasks that are queued by EventSource objects is the remote event task source.").

@jdm could you please confirm that it would indeed be useful to do this at this stage?

Yes, getting the right task sources for the websocket code would be useful!

Will probably finish the pr this weekend.