Get the default device scale factor from monitor

ui/gfx/screen_gtk.cc

 // Copyright (c) 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/gfx/screen.h"
 
 #include <gdk/gdkx.h>
 #include <gtk/gtk.h>
 
 #include "base/logging.h"
@@ skipping 39 matching lines @@
 gfx::Rect NativePrimaryMonitorBounds() {
   GdkScreen* screen = gdk_screen_get_default();
   GdkRectangle rect;
   gdk_screen_get_monitor_geometry(screen, 0, &rect);
   return gfx::Rect(rect);
 }
 
 gfx::Rect GetMonitorAreaNearestWindow(gfx::NativeView view) {
   GdkScreen* screen = gdk_screen_get_default();
   gint monitor_num = 0;
-  if (view) {
+  if (view && GTK_IS_WINDOW(view)) {
     GtkWidget* top_level = gtk_widget_get_toplevel(view);
     DCHECK(GTK_IS_WINDOW(top_level));
     GtkWindow* window = GTK_WINDOW(top_level);
     screen = gtk_window_get_screen(window);
     monitor_num = gdk_screen_get_monitor_at_window(
         screen,
         gtk_widget_get_window(top_level));
   }
   GdkRectangle bounds;
   gdk_screen_get_monitor_geometry(screen, monitor_num, &bounds);
@@ skipping 75 matching lines @@
 
 // static
 int Screen::GetNumMonitors() {
   // This query is kinda bogus for Linux -- do we want number of X screens?
   // The number of monitors Xinerama has?  We'll just use whatever GDK uses.
   GdkScreen* screen = gdk_screen_get_default();
   return gdk_screen_get_n_monitors(screen);
 }
 
 }  // namespace gfx