Switching some interfaces to use std::unique_ptr<>.

This helps show where ownership is transfered between objects.

Specifically, this CL wraps cricket::VideoCapturer, MediaEngineInterface
and DataEngineInterface in unique_ptr.

BUG=None
TBR=magjed@webrtc.org

Review-Url: https://codereview.webrtc.org/2685093002
Cr-Commit-Position: refs/heads/master@{#16548}

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

@@ -27,7 +27,8 @@
   _capturer = new webrtc::AVFoundationVideoCapturer();
   rtc::scoped_refptr<webrtc::VideoTrackSourceInterface> source =
       factory.nativeFactory->CreateVideoSource(
-          _capturer, constraints.nativeConstraints.get());
+          std::unique_ptr<cricket::VideoCapturer>(_capturer),
+          constraints.nativeConstraints.get());
 
   return [super initWithNativeVideoSource:source];
 }