Use actual value of display scale factor from GetMonitorScaleFactor().

ui/aura/window_tree_host.cc

Index: ui/aura/window_tree_host.cc
diff --git a/ui/aura/window_tree_host.cc b/ui/aura/window_tree_host.cc
index ef626257d735d2d0a96387217a894573dfe41d8b..048a9758709fe277ebb46eb9ff6cabda3e24491f 100644
--- a/ui/aura/window_tree_host.cc
+++ b/ui/aura/window_tree_host.cc
@@ -34,11 +34,8 @@ namespace aura {
 const char kWindowTreeHostForAcceleratedWidget[] =
     "__AURA_WINDOW_TREE_HOST_ACCELERATED_WIDGET__";
 
-float GetDeviceScaleFactorFromDisplay(Window* window) {
-  display::Display display =
-      display::Screen::GetScreen()->GetDisplayNearestWindow(window);
-  DCHECK(display.is_valid());
-  return display.device_scale_factor();
+float GetDeviceScaleFactorFromScreen(Window* window) {
+  return display::Screen::GetScreen()->GetScaleFactorForWindow(window);
 }
 
 ////////////////////////////////////////////////////////////////////////////////
@@ -67,7 +64,7 @@ void WindowTreeHost::InitHost() {
 }
 
 void WindowTreeHost::InitCompositor() {
-  compositor_->SetScaleAndSize(GetDeviceScaleFactorFromDisplay(window()),
+  compositor_->SetScaleAndSize(GetDeviceScaleFactorFromScreen(window()),
                                GetBounds().size());
   compositor_->SetRootLayer(window()->layer());
   compositor_->SetDisplayColorSpace(
@@ -277,12 +274,15 @@ void WindowTreeHost::OnHostMoved(const gfx::Point& new_location) {
 }
 
 void WindowTreeHost::OnHostResized(const gfx::Size& new_size) {
+  LOG(ERROR) << "WindowTreeHost::OnHostResized(" << new_size.ToString()
+             << ", DPI = " << GetDeviceScaleFactorFromScreen(window()) << ")";
+
   gfx::Size adjusted_size(new_size);
   adjusted_size.Enlarge(output_surface_padding_.width(),
                         output_surface_padding_.height());
   // The compositor should have the same size as the native root window host.
   // Get the latest scale from display because it might have been changed.
-  compositor_->SetScaleAndSize(GetDeviceScaleFactorFromDisplay(window()),
+  compositor_->SetScaleAndSize(GetDeviceScaleFactorFromScreen(window()),
                                adjusted_size);
 
   gfx::Size layer_size = GetBounds().size();