Make WPT webrtc/simplecall.html pass

The test was failing because passing a null ("end of candidates")
candidate to addIceCandidate is not permitted.

Spec link:

https://www.w3.org/TR/webrtc/#dom-rtcpeerconnection-addicecandidate
https://w3c.github.io/webrtc-pc/#dom-rtcpeerconnection-addicecandidate

Bug: none
Change-Id: I1f22d27393a482a882ebe9735488d8cb723b6e75
Reviewed-on: https://chromium-review.googlesource.com/918662Reviewed-by: Henrik Boström <hbos@chromium.org>
Commit-Queue: Harald Alvestrand <hta@chromium.org>
Cr-Commit-Position: refs/heads/master@{#536977}

third_party/WebKit/LayoutTests/external/wpt/webrtc/simplecall-expected.txt

-This is a testharness.js-based test.
-FAIL Can set up a basic WebRTC call. assert_unreached: WebRTC called error callback for getUserMedia Reached unreachable code
-Harness: the test ran to completion.
-

third_party/WebKit/LayoutTests/external/wpt/webrtc/simplecall.html → third_party/WebKit/LayoutTests/external/wpt/webrtc/simplecall.https.html

@@ -86,11 +86,15 @@ property to true in Firefox.
 
   var onIceCandidateToFirst = test.step_func(function(event) {
     // If event.candidate is null = no more candidates.
-    gSecondConnection.addIceCandidate(event.candidate);
+    if (event.candidate) {
+      gSecondConnection.addIceCandidate(event.candidate);
+    }
   });
 
   var onIceCandidateToSecond = test.step_func(function(event) {
-    gFirstConnection.addIceCandidate(event.candidate);
+    if (event.candidate) {
+      gFirstConnection.addIceCandidate(event.candidate);
+    }
   });
 
   var onRemoteTrack = test.step_func(function(event) {