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

ash/shelf/shelf_layout_manager_unittest.cc

Index: ash/shelf/shelf_layout_manager_unittest.cc
diff --git a/ash/shelf/shelf_layout_manager_unittest.cc b/ash/shelf/shelf_layout_manager_unittest.cc
index 9795e0e525259bd6b3d5debec0047e3f2ea76cdd..ed9bb347725918326d1d3a7f1b2e8069dd5b6b8a 100644
--- a/ash/shelf/shelf_layout_manager_unittest.cc
+++ b/ash/shelf/shelf_layout_manager_unittest.cc
@@ -797,7 +797,7 @@ TEST_F(ShelfLayoutManagerTest, LauncherUpdatedWhenStatusAreaChangesSize) {
 
 // Various assertions around auto-hide.
 TEST_F(ShelfLayoutManagerTest, MAYBE_AutoHide) {
-  aura::RootWindow* root = Shell::GetPrimaryRootWindow();
+  aura::Window* root = Shell::GetPrimaryRootWindow();
   aura::test::EventGenerator generator(root, root);
   generator.MoveMouseTo(0, 0);
 
@@ -974,7 +974,7 @@ TEST_F(ShelfLayoutManagerTest, VisibleWhenLockScreenShowing) {
   EXPECT_EQ(SHELF_AUTO_HIDE, shelf->visibility_state());
   EXPECT_EQ(SHELF_AUTO_HIDE_HIDDEN, shelf->auto_hide_state());
 
-  aura::RootWindow* root = Shell::GetPrimaryRootWindow();
+  aura::Window* root = Shell::GetPrimaryRootWindow();
   // LayoutShelf() forces the animation to completion, at which point the
   // launcher should go off the screen.
   shelf->LayoutShelf();