[SystemDisplayAPI] Observe display change on linux/win/mac as well

chrome/browser/extensions/display_info_provider_win.cc

 // Copyright 2014 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 "chrome/browser/extensions/display_info_provider_win.h"
 
 #include <windows.h>
 
 #include "base/hash.h"
 #include "base/strings/string_number_conversions.h"
 #include "base/strings/utf_string_conversions.h"
 #include "base/win/win_util.h"
 #include "extensions/common/api/system_display.h"
+#include "ui/gfx/screen.h"
 #include "ui/gfx/size.h"
 #include "ui/gfx/win/dpi.h"
 
 namespace extensions {
 
 using core_api::system_display::DisplayUnitInfo;
 
 namespace {
 
 BOOL CALLBACK
@@ skipping 50 matching lines @@
     if (unit->id == all_displays[i]->id) {
       unit->name = all_displays[i]->name;
       unit->dpi_x = all_displays[i]->dpi_x;
       unit->dpi_y = all_displays[i]->dpi_y;
       break;
     }
   }
 }
 
 gfx::Screen* DisplayInfoProviderWin::GetActiveScreen() {
-  return NULL;
+  // TODO(scottmg): native screen is wrong http://crbug.com/133312
+  return gfx::Screen::GetNativeScreen();
 }
 
 // static
 DisplayInfoProvider* DisplayInfoProvider::Create() {
   return new DisplayInfoProviderWin();
 }
 
 }  // namespace extensions