Add DesktopCapturerId and attach it to DesktopFrame

webrtc/modules/desktop_capture/desktop_frame.cc

Index: webrtc/modules/desktop_capture/desktop_frame.cc
diff --git a/webrtc/modules/desktop_capture/desktop_frame.cc b/webrtc/modules/desktop_capture/desktop_frame.cc
index dfc0928b6631b83522e498dd07224c1c59e2bf8b..4136fdd931736e837ce3b0222921a2a5ca70bf8b 100644
--- a/webrtc/modules/desktop_capture/desktop_frame.cc
+++ b/webrtc/modules/desktop_capture/desktop_frame.cc
@@ -26,8 +26,8 @@ DesktopFrame::DesktopFrame(DesktopSize size,
       shared_memory_(shared_memory),
       size_(size),
       stride_(stride),
-      capture_time_ms_(0) {
-}
+      capture_time_ms_(0),
+      capturer_id_(DesktopCapturerId::UNKNOWN) {}
 
 DesktopFrame::~DesktopFrame() {}