Add iOS static library GN build script.

NOTRY=True

BUG=webrtc:6372

Review-Url: https://codereview.webrtc.org/2391123002
Cr-Commit-Position: refs/heads/master@{#14532}

webrtc/sdk/objc/Framework/Classes/RTCVideoTrack.mm

@@ -69,9 +69,9 @@
 - (void)addRenderer:(id<RTCVideoRenderer>)renderer {
   // Make sure we don't have this renderer yet.
   for (RTCVideoRendererAdapter *adapter in _adapters) {
-    // Getting around unused variable error
-    if (adapter.videoRenderer != renderer) {
+    if (adapter.videoRenderer == renderer) {
       NSAssert(NO, @"|renderer| is already attached to this track");
+      return;
     }
   }
   // Create a wrapper that provides a native pointer for us.