Remove a repetition of browser type checks

Depending on the type of browser within which the library is executing,
we take the decision how many times to call getUserMedia (e.g. once for
audio and once for video on Firefox). Upon getting a result of the
call, we used to perform similar browser type checks. I see this as
repetition of logic which makes it difficult to add new browser types
(e.g. React Native). Try to examine the result of the call to
getUserMedia and thus avoid re-checkign the browser type.

modules/RTC/RTCUtils.js

  */
 function handleLocalStream(streams, resolution) {
     var audioStream, videoStream, desktopStream, res = [];
-    // If this is FF, the stream parameter is *not* a MediaStream object, it's
-    // an object with two properties: audioStream, videoStream.
-    if (window.webkitMediaStream) {
+
+    // XXX The function obtainAudioAndVideoPermissions has examined the type of
+    // the browser, its capabilities, etc. and has taken the decision whether to
+    // invoke getUserMedia per device (e.g. Firefox) or once for both audio and
+    // video (e.g. Chrome). In order to not duplicate the logic here, examine
+    // the specified streams and figure out what we've received based on
+    // obtainAudioAndVideoPermissions' decision.
+    if (streams) {
+        // As mentioned above, certian types of browser (e.g. Chrome) support
+        // (with a result which meets our requirements expressed bellow) calling
+        // getUserMedia once for both audio and video.
         var audioVideo = streams.audioVideo;
         if (audioVideo) {
             var audioTracks = audioVideo.getAudioTracks();
                     videoStream.addTrack(videoTracks[j]);
                 }
             }
+        } else {
+          // On other types of browser (e.g. Firefox) we choose (namely,
+          // obtainAudioAndVideoPermissions) to call getUsermedia per device
+          // (type).
+          audioStream = streams.audio;
+          videoStream = streams.video;
         }
-
-        // FIXME Checking streams here is unnecessary because there's
-        // streams.audioVideo above.
-        if (streams)
-            desktopStream = streams.desktopStream;
-
-    }
-    else if (RTCBrowserType.isFirefox() || RTCBrowserType.isTemasysPluginUsed()) {   // Firefox and Temasys plugin
-        if (streams) {
-            audioStream = streams.audio;
-            videoStream = streams.video;
-            desktopStream = streams.desktop;
-        }
+        // Again, different choices on different types of browser.
+        desktopStream = streams.desktopStream || streams.desktop;
     }
 
-    if (desktopStream)
+    if (desktopStream) {
         res.push({
             stream: desktopStream,
             track: desktopStream.getVideoTracks()[0],
             mediaType: MediaType.VIDEO,
             videoType: VideoType.DESKTOP
         });
-
-    if(audioStream)
+    }
+    if (audioStream) {
         res.push({
             stream: audioStream,
             track: audioStream.getAudioTracks()[0],
             mediaType: MediaType.AUDIO,
             videoType: null
         });
-
-    if(videoStream)
+    }
+    if (videoStream) {
         res.push({
             stream: videoStream,
             track: videoStream.getVideoTracks()[0],
             videoType: VideoType.CAMERA,
             resolution: resolution
         });
+    }
 
     return res;
 }