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

ash/display/root_window_transformers.cc

Index: ash/display/root_window_transformers.cc
diff --git a/ash/display/root_window_transformers.cc b/ash/display/root_window_transformers.cc
index 61428397b0549814b5211f1138b320ed39857f98..a698fa4797ab63036001e3f32072a432f98737e3 100644
--- a/ash/display/root_window_transformers.cc
+++ b/ash/display/root_window_transformers.cc
@@ -44,6 +44,10 @@ void RoundNearZero(gfx::Transform* transform) {
   }
 }
 
+// TODO(oshima): Transformers should be able to adjust itself
+// when the device scale factor is changed, instead of
+// precalculating the transform using fixed value.
+
 gfx::Transform CreateRotationTransform(aura::RootWindow* root_window,
                                        const gfx::Display& display) {
   DisplayInfo info =