Move RTCVideoSource to webrtc/api/objc.

webrtc/api/objc/RTCVideoSource.h

Index: webrtc/api/objc/RTCVideoSource.h
diff --git a/webrtc/api/objc/WebRTC-Prefix.pch b/webrtc/api/objc/RTCVideoSource.h
similarity index 71%
copy from webrtc/api/objc/WebRTC-Prefix.pch
copy to webrtc/api/objc/RTCVideoSource.h
index 990b1602da936a98a04b315411b2487aaaf904e5..47895da7a476e99e72e44d38a2ee34f078bdb0b5 100644
--- a/webrtc/api/objc/WebRTC-Prefix.pch
+++ b/webrtc/api/objc/RTCVideoSource.h
@@ -8,6 +8,14 @@
  *  be found in the AUTHORS file in the root of the source tree.
  */
 
-#if !defined(__has_feature) || !__has_feature(objc_arc)
-#error "This file requires ARC support."
-#endif
+#import "RTCMediaSource.h"
+
+NS_ASSUME_NONNULL_BEGIN
+
+@interface RTCVideoSource : RTCMediaSource
+
+- (instancetype)init NS_UNAVAILABLE;
+
+@end
+
+NS_ASSUME_NONNULL_END