[web-bluetooth] Pull Request: Enforce that the current realm is connected while running GATT procedures.

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

== Enforce that the current realm is connected while running GATT 
procedures. ==
This change makes it so that a disconnect() call takes effect 
immediately and
prevents any subsequent changes to member variables from a returning 
GATT
procedure, even if the GATT procedure started before the 
disconnection. It also
forces all active requests to fail with a NetworkError so that no 
remnants of
the connection keep running after it's terminated.

This reverses the readValue() promise and event order from #175, in 
order to
avoid needing to perform a microtask checkpoint inside the algorithm.

Fixes #114 and #171.

Preview at 
https://rawgit.com/jyasskin/web-bluetooth-1/enforce-connectedness/index.html.

See https://github.com/WebBluetoothCG/web-bluetooth/pull/209