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

webrtc/modules/desktop_capture/cropped_desktop_frame.h

 /*
  *  Copyright (c) 2014 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.
  */
 
 #ifndef WEBRTC_MODULES_DESKTOP_CAPTURE_CROPPED_DESKTOP_FRAME_H_
 #define WEBRTC_MODULES_DESKTOP_CAPTURE_CROPPED_DESKTOP_FRAME_H_
 
 #include "webrtc/modules/desktop_capture/desktop_frame.h"
 
 namespace webrtc {
 
-// Always takes ownership of |frame|. Returns NULL if |rect| is not contained
-// by the bounds of |frame|.
-DesktopFrame* CreateCroppedDesktopFrame(DesktopFrame* frame,
-                                        const DesktopRect& rect);
+// Returns nullptr frame if |rect| is not contained by the bounds of |frame|.
+std::unique_ptr<DesktopFrame> CreateCroppedDesktopFrame(
+    std::unique_ptr<DesktopFrame> frame,
+    const DesktopRect& rect);
+
 }  // namespace webrtc
 
 #endif  // WEBRTC_MODULES_DESKTOP_CAPTURE_CROPPED_DESKTOP_FRAME_H_