examples: dont fiddle with srcObject if already set

dont attempt to set srcObject if it is already set in
the examples.

This covers two cases:
* ontrack with an audio and a video track
* early media (see discussion in #1128)

webrtc.html

@@ -10954,9 +10954,9 @@ <h3>Simple Peer-to-peer Example</h3>
         .catch(logError);
     };
 
-    // once remote video track arrives, show it in the remote video element
+    // once remote track arrives, show it in the remote video element
     pc.ontrack = function (evt) {
-        if (evt.track.kind === "video")
+        if (!remoteView.srcObject)
           remoteView.srcObject = evt.streams[0];
     };
 
@@ -11049,7 +11049,7 @@ <h3>Advanced Peer-to-peer Example with Warm-up</h3>
         .catch(logError);
     };
 
-    // once remote video track arrives, show it in the remote video element
+    // once remote track arrives, show it in the remote video element
     pc.ontrack = function (evt) {
         if (evt.track.kind === "audio") {
           if (answerer) {
@@ -11067,8 +11067,9 @@ <h3>Advanced Peer-to-peer Example with Warm-up</h3>
               video.sender.replaceTrack(videoSendTrack);
             }
           }
-          remoteView.srcObject = evt.streams[0];
         }
+        if (!remoteView.srcObject)
+          remoteView.srcObject = evt.streams[0];
     };
 
     // get a local stream, show it in a self-view and add it to be sent