Fails when called from worker

[jackkleeman]

This function seems to expect document and window globals to exist, giving a runtime error when called in a web worker. As far as I know, webrtc support does not exist within a worker, so a simple fix is to check if document and window exist, and if not then return false for all relevant booleans.

[daviddias]

Yeah, this is the last task to get ipfs/js-ipfs#725 done too :)