This change implements following behavior defined in the spec:
https://html.spec.whatwg.org/multipage/web-messaging.html#examples-5
> The start() method, whether called explicitly or implicitly (by
setting onmessage), starts the flow of messages: messages posted on
message ports are initially paused, so that they don't get dropped on
the floor before the script has had a chance to set up its handlers.
Now we don't read bytes from the underlying transport socket until the
message port transitions to the enabled state. This required the
following places to explicitly enable the message port, because now,
when it actually matters, we must do so, or otherwise sent messages will
get stuck:
- `onmessage` attribute setter in DedicatedWorkerGlobalScope, because
it implicitly sets the onmessage handler for the worker's underlying
port.
- Stream API operations where the message port enabling steps were
previously marked as FIXMEs.
Also push the onconnect event for the initial connection.
This still doesn't properly handle sending an onconnect event to a
pre-existing SharedWorkerGlobalScope with the same name for the same
origin, but it does give us a lot of WPT passes in the SharedWorker
category.
