Implement scoped_const_refptr template.

webrtc/modules/desktop_capture/x11/shared_x_display.h

Index: webrtc/modules/desktop_capture/x11/shared_x_display.h
diff --git a/webrtc/modules/desktop_capture/x11/shared_x_display.h b/webrtc/modules/desktop_capture/x11/shared_x_display.h
index 6cade2decc09e3cc9541d5d3dd9e74f6bee7f108..76384a1c9efd288c3120eb2bc19e5f4f3db39edc 100644
--- a/webrtc/modules/desktop_capture/x11/shared_x_display.h
+++ b/webrtc/modules/desktop_capture/x11/shared_x_display.h
@@ -49,8 +49,8 @@ class SharedXDisplay {
   // DISPLAY). NULL is returned if X11 connection failed.
   static rtc::scoped_refptr<SharedXDisplay> CreateDefault();
 
-  void AddRef() { ++ref_count_; }
-  void Release() {
+  void AddRef() const { ++ref_count_; }
+  void Release() const {
     if (--ref_count_ == 0)
       delete this;
   }
@@ -72,7 +72,7 @@ class SharedXDisplay {
 
   ~SharedXDisplay();
 
-  Atomic32 ref_count_;
+  mutable Atomic32 ref_count_;
   Display* display_;
 
   EventHandlersMap event_handlers_;