Create RootWindowTransformer after root window's bounds and scale factor are updated.

ash/display/display_manager_unittest.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/display/display_manager.h"
 
 #include "ash/display/display_controller.h"
 #include "ash/screen_ash.h"
 #include "ash/shell.h"
 #include "ash/test/ash_test_base.h"
@@ skipping 356 matching lines @@
   EXPECT_EQ(1u, changed().size());
   EXPECT_EQ(display2_id, changed()[0].id());
 
   reset();
   display_manager()->SetOverscanInsets(display2_id, gfx::Insets(0, 0, 0, 0));
   EXPECT_EQ(1u, changed().size());
   EXPECT_EQ(display2_id, changed()[0].id());
 }
 
 TEST_F(DisplayManagerTest, TestDeviceScaleOnlyChange) {
+  if (!SupportsHostWindowResize())
+    return;
+
   UpdateDisplay("1000x600");
   EXPECT_EQ(1,
             Shell::GetPrimaryRootWindow()->compositor()->device_scale_factor());
   EXPECT_EQ("1000x600",
             Shell::GetPrimaryRootWindow()->bounds().size().ToString());
   UpdateDisplay("1000x600*2");
   EXPECT_EQ(2,
             Shell::GetPrimaryRootWindow()->compositor()->device_scale_factor());
   EXPECT_EQ("500x300",
             Shell::GetPrimaryRootWindow()->bounds().size().ToString());
@@ skipping 608 matching lines @@
   UpdateDisplay("400x600/o,600x800/o");
   EXPECT_FALSE(display_observer.changed_and_reset());
   EXPECT_EQ("400x600", test_api.GetRootWindow()->bounds().size().ToString());
   EXPECT_EQ("600x800", GetMirroredDisplay().size().ToString());
 
   Shell::GetScreen()->RemoveObserver(&display_observer);
 }
 
 }  // namespace internal
 }  // namespace ash