[ServiceWorker] "javascript global environment" and related terms were removed from HTML (#832)

whatwg/html@cf0355d7e0e229b98f7fbd51b8c7608010c787f5 removed definitions of "document environment", "worker environment" and "javascript global environment", which this spec still links to.

@domenic Not sure what best to replace them with (but maybe fixing #831 is partly related to this?)

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Reply to this email directly or view it on GitHub:
https://github.com/slightlyoff/ServiceWorker/issues/832