[web-bluetooth] Pull Request: Remove all mentions of "script execution environment", which no longer exists in HTML.

jyasskin has just submitted a new pull request for 
https://github.com/WebBluetoothCG/web-bluetooth:

== Remove all mentions of "script execution environment", which no 
longer exists in HTML. ==
Preview at 
https://rawgit.com/jyasskin/web-bluetooth-1/remove-script-execution-environment/index.html

@bzbarsky It looks like other specs leave platform objects' global 
environments implied rather than explicitly wired, so I'm following 
that convention here. I wound up using internal slots on the 
`Bluetooth` instance in nearly all cases, so I only needed to use that
 implied environment in one place: in 
https://rawgit.com/jyasskin/web-bluetooth-1/remove-script-execution-environment/index.html#service-change-events
 to find an event loop.

See https://github.com/WebBluetoothCG/web-bluetooth/pull/198