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

ash/wm/overview/scoped_transform_overview_window.cc

Index: ash/wm/overview/scoped_transform_overview_window.cc
diff --git a/ash/wm/overview/scoped_transform_overview_window.cc b/ash/wm/overview/scoped_transform_overview_window.cc
index e851516f583866cf006ed46e8ba4d472de52de1c..49fd8e8c4eedbd1fb6895745c611e1ad9a37ad99 100644
--- a/ash/wm/overview/scoped_transform_overview_window.cc
+++ b/ash/wm/overview/scoped_transform_overview_window.cc
@@ -9,7 +9,6 @@
 #include "ash/wm/window_state.h"
 #include "ui/aura/client/aura_constants.h"
 #include "ui/aura/client/screen_position_client.h"
-#include "ui/aura/root_window.h"
 #include "ui/aura/window.h"
 #include "ui/compositor/layer_animation_observer.h"
 #include "ui/compositor/scoped_layer_animation_settings.h"
@@ -26,7 +25,7 @@ namespace ash {
 namespace {
 
 // Creates a copy of |window| with |recreated_layer| in the |target_root|.
-views::Widget* CreateCopyOfWindow(aura::RootWindow* target_root,
+views::Widget* CreateCopyOfWindow(aura::Window* target_root,
                                   aura::Window* src_window,
                                   ui::Layer* recreated_layer) {
   // Save and remove the transform from the layer to later reapply to both the
@@ -317,7 +316,7 @@ gfx::Transform ScopedTransformOverviewWindow::GetTransformForRect(
 }
 
 void ScopedTransformOverviewWindow::SetTransform(
-    aura::RootWindow* root_window,
+    aura::Window* root_window,
     const gfx::Transform& transform,
     bool animate) {
   DCHECK(overview_started_);