Adding stub files needed for https://codereview.webrtc.org/1507973003/

talk/app/webrtc/mediastreamobserver.h

Index: talk/app/webrtc/mediastreamobserver.h
diff --git a/talk/app/webrtc/objc/RTCAVFoundationVideoSource+Internal.h b/talk/app/webrtc/mediastreamobserver.h
similarity index 86%
copy from talk/app/webrtc/objc/RTCAVFoundationVideoSource+Internal.h
copy to talk/app/webrtc/mediastreamobserver.h
index ffa6629c41cabb77005d43a929adcb9d49cb34a9..d6128f397338c67c31556e04c3e98a888f2a84a4 100644
--- a/talk/app/webrtc/objc/RTCAVFoundationVideoSource+Internal.h
+++ b/talk/app/webrtc/mediastreamobserver.h
@@ -25,12 +25,9 @@
  * ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
  */
 
-#import "RTCAVFoundationVideoSource.h"
+// This file is currently stubbed so that Chromium's build files can be updated.
 
-#include "avfoundationvideocapturer.h"
+#ifndef TALK_APP_WEBRTC_MEDIASTREAMOBSERVER_H_
+#define TALK_APP_WEBRTC_MEDIASTREAMOBSERVER_H_
 
-@interface RTCAVFoundationVideoSource ()
-
-@property(nonatomic, readonly) webrtc::AVFoundationVideoCapturer* capturer;
-
-@end
+#endif  // TALK_APP_WEBRTC_MEDIASTREAMOBSERVER_H_