[ServiceWorker] Association between a service worker and global objects (#652)

In section 4.2.4 step 9.4, the source of the MessageEvent is set to a ServiceWorker object that represents the service worker associated with the global object specified by the incumbent settings object.
How is a service worker associated to the global object?

cc @annevk 

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Reply to this email directly or view it on GitHub:
https://github.com/slightlyoff/ServiceWorker/issues/652