UserAgent: Mozilla/5.0 (X11; Linux x86_64; rv:54.0) Gecko/20100101 Firefox/54.0

Steps to reproduce the problem:
1. Open a RTCPeerConnection
2. call close on the object
3. call close again

What is the expected behavior?
Second call to close should have no effect. Spec*) says in point 2:

If connection's [[isClosed]] slot is true, abort these steps.

*) http://w3c.github.io/webrtc-pc/#dom-rtcpeerconnection-close

What went wrong?
Application crashes with uncaught exception:

```
Error: Failed to execute 'close' on 'RTCPeerConnection': The RTCPeerConnection's signalingState is 'closed'.
```

Did this work before? No 

Does this work in other browsers? Yes

Chrome version: 58.0.3029.110 (Developer Build) (64-bit)  Channel: n/a
OS Version: nixos
Flash Version: 

Firefox works as expected and ignores the second call to close().