Add CreateWindowCapturer() and CreateScreenCapturer() in DesktopCapturer

webrtc/modules/desktop_capture/screen_capturer_null.cc

Index: webrtc/modules/desktop_capture/screen_capturer_null.cc
diff --git a/webrtc/modules/desktop_capture/screen_capturer_null.cc b/webrtc/modules/desktop_capture/screen_capturer_null.cc
index a0bc7f13ea2329bae1b95bad7e46e87e716094da..320a01ce4bba5b7493ebab5dc8c385dab8dd6c86 100644
--- a/webrtc/modules/desktop_capture/screen_capturer_null.cc
+++ b/webrtc/modules/desktop_capture/screen_capturer_null.cc
@@ -17,4 +17,10 @@ ScreenCapturer* ScreenCapturer::Create(const DesktopCaptureOptions& options) {
   return NULL;
 }
 
+// static
+std::unique_ptr<DesktopCapturer> DesktopCapturer::CreateRawScreenCapturer(
+    const DesktopCaptureOptions& options) {
+  return nullptr;
+}
+
 }  // namespace webrtc