Rewrite examples to avoid onnegotiationneeded

onnegotiationneeded is broken in libwebrtc M73, so
just rewrite all examples to only set the offer once

Resolves #545

examples/save-to-disk/jsfiddle/demo.js

@@ -12,8 +12,10 @@ var log = msg => {
 }
 
 navigator.mediaDevices.getUserMedia({ video: true, audio: true })
-  .then(stream => pc.addStream(document.getElementById('video1').srcObject = stream))
-  .catch(log)
+  .then(stream => {
+    pc.addStream(document.getElementById('video1').srcObject = stream)
+    pc.createOffer().then(d => pc.setLocalDescription(d)).catch(log)
+  }).catch(log)
 
 pc.oniceconnectionstatechange = e => log(pc.iceConnectionState)
 pc.onicecandidate = event => {
@@ -22,9 +24,6 @@ pc.onicecandidate = event => {
   }
 }
 
-pc.onnegotiationneeded = e =>
-  pc.createOffer().then(d => pc.setLocalDescription(d)).catch(log)
-
 window.startSession = () => {
   let sd = document.getElementById('remoteSessionDescription').value
   if (sd === '') {