ObjC: RTCVideoSource cleanup

RTCVideoSource was recently added in
https://codereview.webrtc.org/2745193002/. This CL addresses some post
commit feedback.

BUG=webrtc:7177

Review-Url: https://codereview.webrtc.org/2812533003
Cr-Commit-Position: refs/heads/master@{#17649}
diff --git a/webrtc/sdk/objc/Framework/Classes/RTCVideoCapturer.m b/webrtc/sdk/objc/Framework/Classes/RTCVideoCapturer.m
index e0a307b..9d4d99d 100644
--- a/webrtc/sdk/objc/Framework/Classes/RTCVideoCapturer.m
+++ b/webrtc/sdk/objc/Framework/Classes/RTCVideoCapturer.m
@@ -10,19 +10,16 @@
 
 #import "WebRTC/RTCVideoCapturer.h"
 
-@implementation RTCVideoCapturer {
-  __weak id<RTCVideoCapturerDelegate> _delegate;
-}
+@implementation RTCVideoCapturer
+
+@synthesize delegate = _delegate;
 
 - (instancetype)initWithDelegate:(id<RTCVideoCapturerDelegate>)delegate {
+  NSAssert(delegate != nil, @"delegate cannot be nil");
   if (self = [super init]) {
     _delegate = delegate;
   }
   return self;
 }
 
-- (id<RTCVideoCapturerDelegate>)delegate {
-  return _delegate;
-}
-
 @end