Explicitly retain self in objc blocks to avoid compiler warning.

Implicitly retaining self pointer (assuming this is intended behavior) causes compiler warning `-Wimplicit-retain-self`. We should do it explicitly.

Bug: webrtc:9971
Change-Id: If77a67168d8a65ced78d5119b9a7332391d20bc9
Reviewed-on: https://webrtc-review.googlesource.com/c/109641
Commit-Queue: Jiawei Ou <ouj@fb.com>
Reviewed-by: Kári Helgason <kthelgason@webrtc.org>
Reviewed-by: Tommi <tommi@webrtc.org>
Cr-Commit-Position: refs/heads/master@{#25609}

examples/objc/AppRTCMobile/ios/ARDVideoCallViewController.m

@@ -27,6 +27,7 @@
                                           RTCAudioSessionDelegate>
 @property(nonatomic, strong) RTCVideoTrack *remoteVideoTrack;
 @property(nonatomic, readonly) ARDVideoCallView *videoCallView;
+@property(nonatomic, assign) AVAudioSessionPortOverride portOverride;
 @end
 
 @implementation ARDVideoCallViewController {
@@ -34,12 +35,12 @@
   RTCVideoTrack *_remoteVideoTrack;
   ARDCaptureController *_captureController;
   ARDFileCaptureController *_fileCaptureController NS_AVAILABLE_IOS(10);
-  AVAudioSessionPortOverride _portOverride;
 }
 
 @synthesize videoCallView = _videoCallView;
 @synthesize remoteVideoTrack = _remoteVideoTrack;
 @synthesize delegate = _delegate;
+@synthesize portOverride = _portOverride;
 
 - (instancetype)initForRoom:(NSString *)room
                  isLoopback:(BOOL)isLoopback
@@ -168,7 +169,7 @@
     [session lockForConfiguration];
     NSError *error = nil;
     if ([session overrideOutputAudioPort:override error:&error]) {
-      _portOverride = override;
+      self.portOverride = override;
     } else {
       RTCLogError(@"Error overriding output port: %@",
                   error.localizedDescription);