Use std::unique_ptr<> to pass frame ownership in DesktopCapturer impls.

webrtc/modules/desktop_capture/window_capturer_null.cc

 /*
  *  Copyright (c) 2013 The WebRTC project authors. All Rights Reserved.
  *
  *  Use of this source code is governed by a BSD-style license
  *  that can be found in the LICENSE file in the root of the source
  *  tree. An additional intellectual property rights grant can be found
  *  in the file PATENTS.  All contributing project authors may
  *  be found in the AUTHORS file in the root of the source tree.
  */
 
@@ skipping 16 matching lines @@
   // WindowCapturer interface.
   bool GetWindowList(WindowList* windows) override;
   bool SelectWindow(WindowId id) override;
   bool BringSelectedWindowToFront() override;
 
   // DesktopCapturer interface.
   void Start(Callback* callback) override;
   void Capture(const DesktopRegion& region) override;
 
  private:
-  Callback* callback_;
+  Callback* callback_ = nullptr;
 
   RTC_DISALLOW_COPY_AND_ASSIGN(WindowCapturerNull);
 };
 
-WindowCapturerNull::WindowCapturerNull()
-    : callback_(NULL) {
-}
-
-WindowCapturerNull::~WindowCapturerNull() {
-}
+WindowCapturerNull::WindowCapturerNull() {}
+WindowCapturerNull::~WindowCapturerNull() {}
 
 bool WindowCapturerNull::GetWindowList(WindowList* windows) {
   // Not implemented yet.
   return false;
 }
 
 bool WindowCapturerNull::SelectWindow(WindowId id) {
   // Not implemented yet.
   return false;
 }
 
 bool WindowCapturerNull::BringSelectedWindowToFront() {
   // Not implemented yet.
   return false;
 }
 
 void WindowCapturerNull::Start(Callback* callback) {
   assert(!callback_);
   assert(callback);
 
   callback_ = callback;
 }
 
 void WindowCapturerNull::Capture(const DesktopRegion& region) {
   // Not implemented yet.
-  callback_->OnCaptureCompleted(NULL);
+  callback_->OnCaptureCompleted(std::unique_ptr<DesktopFrame>());
 }
 
 }  // namespace
 
 // static
 WindowCapturer* WindowCapturer::Create(const DesktopCaptureOptions& options) {
   return new WindowCapturerNull();
 }
 
 }  // namespace webrtc