webrtc: Fixes WebRtcBrowserTest to use DOM attributes correctly.

WebRtcBrowserTest.RunsAudioVideoWebRTCCallInTwoTabs is assuming that DOM attributes are own properties on a JS obejct, which is not correct.  Blink (binding) team is working to move DOM attributes from own properties to properties on prototype chains, and then this test will break.

This CL fixes the test so that it stringifies not only own properties but also attributes on prototype chains.

BUG=43394

Review URL: https://codereview.chromium.org/904823002

Cr-Commit-Position: refs/heads/master@{#315198}

chrome/test/data/webrtc/peerconnection.js

@@ -57,7 +57,7 @@ function createLocalOffer(constraints) {
         success_('createOffer');
         setLocalDescription(peerConnection, localOffer);
 
-        returnToTest('ok-' + JSON.stringify(localOffer));
+        returnToTest('ok-' + stringifyDOMObject_(localOffer));
       },
       function(error) { failure_('createOffer', error); },
       constraints);
@@ -89,7 +89,7 @@ function receiveOfferFromPeer(sessionDescJson, constraints) {
       function(answer) {
         success_('createAnswer');
         setLocalDescription(peerConnection, answer);
-        returnToTest('ok-' + JSON.stringify(answer));
+        returnToTest('ok-' + stringifyDOMObject_(answer));
       },
       function(error) { failure_('createAnswer', error); },
       constraints);
@@ -178,7 +178,7 @@ function getAllIceCandidates() {
     return;
   }
 
-  returnToTest(JSON.stringify(gIceCandidates));
+  returnToTest(stringifyDOMObject_(gIceCandidates));
 }
 
 /**
@@ -245,7 +245,7 @@ function success_(method) {
 
 /** @private */
 function failure_(method, error) {
-  throw failTest(method + '() failed: ' + JSON.stringify(error));
+  throw failTest(method + '() failed: ' + stringifyDOMObject_(error));
 }
 
 /** @private */
@@ -279,6 +279,28 @@ function removeStreamCallback_(event) {
   document.getElementById('remote-view').src = '';
 }
 
+/**
+ * Stringifies a DOM object.
+ *
+ * This function stringifies not only own properties but also DOM attributes
+ * which are on a prototype chain.  Note that JSON.stringify only stringifies
+ * own properties.
+ * @private
+ */
+function stringifyDOMObject_(object)
+{
+  function deepCopy(src) {
+    if (typeof src != "object")
+      return src;
+    var dst = Array.isArray(src) ? [] : {};
+    for (var property in src) {
+      dst[property] = deepCopy(src[property]);
+    }
+    return dst;
+  }
+  return JSON.stringify(deepCopy(object));
+}
+
 /**
  * Parses JSON-encoded session descriptions and ICE candidates.
  * @private