Replace scoped_ptr with unique_ptr everywhere

But keep #including scoped_ptr.h in .h files, so as not to break
WebRTC users who expect those .h files to give them rtc::scoped_ptr.

BUG=webrtc:5520

Review-Url: https://codereview.webrtc.org/1937693002
Cr-Commit-Position: refs/heads/master@{#12581}

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

@@ -10,10 +10,10 @@
 
 #import "RTCVideoFrame+Private.h"
 
-#include "webrtc/base/scoped_ptr.h"
+#include <memory>
 
 @implementation RTCVideoFrame {
-  rtc::scoped_ptr<cricket::VideoFrame> _videoFrame;
+  std::unique_ptr<cricket::VideoFrame> _videoFrame;
   rtc::scoped_refptr<webrtc::VideoFrameBuffer> _i420Buffer;
 }