[xhr] Remove XMLHttpRequest from ServiceWorkerGlobalScope (#19)

I noticed a discrepancy between the spec IDL and Blink, that comes down to this bug:
https://code.google.com/p/chromium/issues/detail?id=395931

If this makes sense, I suggest replacing `Exposed=(Window,Worker)` with `Exposed=(Window,DedicatedWorker,SharedWorker)` in the spec too.

CC @coonsta and @jakearchibald

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Reply to this email directly or view it on GitHub:
https://github.com/whatwg/xhr/issues/19