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

webrtc/modules/desktop_capture/window_capturer_null.cc

Index: webrtc/modules/desktop_capture/window_capturer_null.cc
diff --git a/webrtc/modules/desktop_capture/window_capturer_null.cc b/webrtc/modules/desktop_capture/window_capturer_null.cc
index 5f32c3d71ccf4f51e2217205d4dcd8f680c94bec..3fe21cf7189322e1c1e5fb0d516ba531d8a1bfb6 100755
--- a/webrtc/modules/desktop_capture/window_capturer_null.cc
+++ b/webrtc/modules/desktop_capture/window_capturer_null.cc
@@ -34,17 +34,13 @@ class WindowCapturerNull : public WindowCapturer {
   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.
@@ -70,7 +66,7 @@ void WindowCapturerNull::Start(Callback* callback) {
 
 void WindowCapturerNull::Capture(const DesktopRegion& region) {
   // Not implemented yet.
-  callback_->OnCaptureCompleted(NULL);
+  callback_->OnCaptureResult(Result::ERROR_TEMPORARY, nullptr);
 }
 
 }  // namespace