webrtc wpt: close RTCPeerConnection in generateAnswer helper

BUG=836871

Change-Id: Idd0495f538777469043fec30993f5e9399d62c68
Reviewed-on: https://chromium-review.googlesource.com/1140676Reviewed-by: Henrik Boström <hbos@chromium.org>
Commit-Queue: Philipp Hancke <philipp.hancke@googlemail.com>
Cr-Commit-Position: refs/heads/master@{#577457}

third_party/WebKit/LayoutTests/external/wpt/webrtc/RTCPeerConnection-helper.js

@@ -165,7 +165,11 @@ function generateOffer(options={}) {
 function generateAnswer(offer) {
   const pc = new RTCPeerConnection();
   return pc.setRemoteDescription(offer)
-  .then(() => pc.createAnswer());
+  .then(() => pc.createAnswer())
+  .then((answer) => {
+    pc.close();
+    return answer;
+  });
 }
 
 // Run a test function that return a promise that should