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

ash/display/display_controller_unittest.cc

Index: ash/display/display_controller_unittest.cc
diff --git a/ash/display/display_controller_unittest.cc b/ash/display/display_controller_unittest.cc
index 06168daf010a66b9b88dbe808d82dbe5244e5f63..374c025f34621bc327afdfd9be308ff995bdf1d7 100644
--- a/ash/display/display_controller_unittest.cc
+++ b/ash/display/display_controller_unittest.cc
@@ -753,6 +753,20 @@ TEST_F(DisplayControllerTest, OverscanInsets) {
   generator.MoveMouseToInHost(30, 20);
   EXPECT_EQ("30,20", event_handler.GetLocationAndReset());
 
+
+  // Make sure the root window transformer uses correct scale
+  // factor when swapping display. Test crbug.com/253690.
+  UpdateDisplay("400x300*2,600x400/o");
+  root_windows = Shell::GetAllRootWindows();
+  gfx::Point point;
+  Shell::GetAllRootWindows()[1]->GetRootTransform().TransformPoint(point);
+  EXPECT_EQ("15,10", point.ToString());
+
+  display_controller->SwapPrimaryDisplay();
+  point.SetPoint(0, 0);
+  Shell::GetAllRootWindows()[1]->GetRootTransform().TransformPoint(point);
+  EXPECT_EQ("15,10", point.ToString());
+
   Shell::GetInstance()->RemovePreTargetHandler(&event_handler);
 }