Wrap ScreenCapturer with ScreenCapturerDifferWrapper

webrtc/modules/desktop_capture/win/screen_capturer_win_directx.h

Index: webrtc/modules/desktop_capture/win/screen_capturer_win_directx.h
diff --git a/webrtc/modules/desktop_capture/win/screen_capturer_win_directx.h b/webrtc/modules/desktop_capture/win/screen_capturer_win_directx.h
index 119ffaecdfb0abac84436cca9caa33837129fa47..9c25c711b6630b4f2b5021d9cb68ac7a1e06df78 100644
--- a/webrtc/modules/desktop_capture/win/screen_capturer_win_directx.h
+++ b/webrtc/modules/desktop_capture/win/screen_capturer_win_directx.h
@@ -28,10 +28,14 @@ namespace webrtc {
 // implementation won't work when ScreenCaptureFrameQueue.kQueueLength is not 2.
 class ScreenCapturerWinDirectx : public ScreenCapturer {
  public:
-  // Whether the system support DirectX based capturing.
+  // Whether the system supports DirectX based capturing.
   static bool IsSupported();
 
-  explicit ScreenCapturerWinDirectx(const DesktopCaptureOptions& options);
+  // Creates a ScreenCapturer which uses DirectX APIs to capture screen. Returns
+  // nullptr if the system does not support DirectX duplication APIs.
+  static std::unique_ptr<ScreenCapturer> Create(
+      const DesktopCaptureOptions& options);
+
   virtual ~ScreenCapturerWinDirectx();
 
   void Start(Callback* callback) override;
@@ -42,6 +46,8 @@ class ScreenCapturerWinDirectx : public ScreenCapturer {
   bool SelectScreen(ScreenId id) override;
 
  private:
+  explicit ScreenCapturerWinDirectx(const DesktopCaptureOptions& options);
+
   // Returns desktop size of selected screen.
   DesktopSize SelectedDesktopSize() const;