Add CreateWindowCapturer() and CreateScreenCapturer() in DesktopCapturer

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 78e5bea7bcc6723979377ca7f76e61b031d99f1c..3afe533e2b088631a841e1e07248266b8db6e604 100755
--- a/webrtc/modules/desktop_capture/window_capturer_null.cc
+++ b/webrtc/modules/desktop_capture/window_capturer_null.cc
@@ -76,4 +76,10 @@ WindowCapturer* WindowCapturer::Create(const DesktopCaptureOptions& options) {
   return new WindowCapturerNull();
 }
 
+// static
+std::unique_ptr<DesktopCapturer> DesktopCapturer::CreateRawWindowCapturer(
+    const DesktopCaptureOptions& options) {
+  return std::unique_ptr<DesktopCapturer>(new WindowCapturerNull());
+}
+
 }  // namespace webrtc