Desktop aura: Break aura::Window::SetParent in two.

ash/drag_drop/drag_drop_tracker.h

Index: ash/drag_drop/drag_drop_tracker.h
diff --git a/ash/drag_drop/drag_drop_tracker.h b/ash/drag_drop/drag_drop_tracker.h
index 6493b06130c893c672df5f734ff643b809e52fd6..22cf630261b7ac55a9ffd19800c697169c8210f3 100644
--- a/ash/drag_drop/drag_drop_tracker.h
+++ b/ash/drag_drop/drag_drop_tracker.h
@@ -11,6 +11,7 @@
 #include "ui/base/events/event.h"
 
 namespace aura {
+class RootWindow;
 class Window;
 }
 
@@ -24,7 +25,7 @@ namespace internal {
 // is supported for now.
 class ASH_EXPORT DragDropTracker {
  public:
-  DragDropTracker();
+  explicit DragDropTracker(aura::RootWindow* context_root);
   ~DragDropTracker();
 
   aura::Window* capture_window() { return capture_window_.get(); }