Make GetRootWindow() return a Window instead of a RootWindow.

ash/drag_drop/drag_drop_tracker.cc

 // Copyright (c) 2012 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 #include "ash/drag_drop/drag_drop_tracker.h"
 
 #include "ash/shell.h"
 #include "ash/shell_window_ids.h"
 #include "ash/wm/coordinate_conversion.h"
 #include "ui/aura/client/window_tree_client.h"
@@ skipping 32 matching lines @@
 
 void DragDropTracker::TakeCapture() {
   capture_window_->SetCapture();
 }
 
 aura::Window* DragDropTracker::GetTarget(const ui::LocatedEvent& event) {
   DCHECK(capture_window_.get());
   gfx::Point location_in_screen = event.location();
   wm::ConvertPointToScreen(capture_window_.get(),
                            &location_in_screen);
-  aura::RootWindow* root_window_at_point =
+  aura::Window* root_window_at_point =
       wm::GetRootWindowAt(location_in_screen);
   gfx::Point location_in_root = location_in_screen;
   wm::ConvertPointFromScreen(root_window_at_point, &location_in_root);
   return root_window_at_point->GetEventHandlerForPoint(location_in_root);
 }
 
 ui::LocatedEvent* DragDropTracker::ConvertEvent(
     aura::Window* target,
     const ui::LocatedEvent& event) {
   DCHECK(capture_window_.get());
   gfx::Point target_location = event.location();
   aura::Window::ConvertPointToTarget(capture_window_.get(), target,
                                      &target_location);
   gfx::Point location_in_screen = event.location();
   ash::wm::ConvertPointToScreen(capture_window_.get(), &location_in_screen);
   gfx::Point target_root_location = event.root_location();
   aura::Window::ConvertPointToTarget(
       capture_window_->GetRootWindow(),
       ash::wm::GetRootWindowAt(location_in_screen),
       &target_root_location);
   return new ui::MouseEvent(event.type(),
                             target_location,
                             target_root_location,
                             event.flags());
 }
 
 }  // namespace internal
 }  // namespace ash