[web-bluetooth] Serialize start/stop notifications

jyasskin has just created a new issue for 
https://github.com/WebBluetoothCG/web-bluetooth:

== Serialize start/stop notifications ==
Right now, 
[startNotifications()](https://webbluetoothcg.github.io/web-bluetooth/#dom-bluetoothgattcharacteristic-startnotifications)
 and 
[stopNotifications()](https://webbluetoothcg.github.io/web-bluetooth/#dom-bluetoothgattcharacteristic-stopnotifications)
 run all of their steps in parallel, meaning that

```javascript
characteristic.startNotifications();
characteristic.stopNotifications();
```

can leave the notifications enabled. We should find wording to exclude
 that race.

See https://github.com/WebBluetoothCG/web-bluetooth/issues/166

Received on Tuesday, 22 September 2015 00:29:01 UTC