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

ui/display/screen.cc

 // Copyright 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 "ui/display/screen.h"
 
 #include "ui/display/display.h"
 #include "ui/gfx/geometry/rect.h"
 
 namespace display {
@@ skipping 28 matching lines @@
   float scale = GetDisplayNearestWindow(view).device_scale_factor();
   return ScaleToEnclosingRect(screen_rect, 1.0f / scale);
 }
 
 gfx::Rect Screen::DIPToScreenRectInWindow(gfx::NativeView view,
                                           const gfx::Rect& dip_rect) const {
   float scale = GetDisplayNearestWindow(view).device_scale_factor();
   return ScaleToEnclosingRect(dip_rect, scale);
 }
 
+float Screen::GetScaleFactorForWindow(gfx::NativeView view) const {
+  const display::Display& nearest_display = GetDisplayNearestWindow(view);
+  DCHECK(nearest_display.is_valid());
+  return nearest_display.device_scale_factor();
+}
+
 }  // namespace display