[push-api] PushMessageData.arrayBuffer() should throw a RangeError when allocation fails (#169)

The ECMAScript specification states that array buffer allocation failures throw a RangeError exception.
The spec for PushMessageData.arrayBuffer() should probably state that it re-throws exceptions that are thrown by arrayBuffer creation. 

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Reply to this email directly or view it on GitHub:
https://github.com/w3c/push-api/issues/169