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

ash/display/display_manager_unittest.cc

Index: ash/display/display_manager_unittest.cc
diff --git a/ash/display/display_manager_unittest.cc b/ash/display/display_manager_unittest.cc
index f1fc5dec168808cb1dd279fcc3e346535f5d0d5f..bfc87e4f6f2888e2cef0b4091f7b51556263c07a 100644
--- a/ash/display/display_manager_unittest.cc
+++ b/ash/display/display_manager_unittest.cc
@@ -374,6 +374,9 @@ TEST_F(DisplayManagerTest, ZeroOverscanInsets) {
 }
 
 TEST_F(DisplayManagerTest, TestDeviceScaleOnlyChange) {
+  if (!SupportsHostWindowResize())
+    return;
+
   UpdateDisplay("1000x600");
   EXPECT_EQ(1,
             Shell::GetPrimaryRootWindow()->compositor()->device_scale_factor());