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

chrome/browser/extensions/display_info_provider_mac.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_mac.h"
 
+#include "ui/gfx/screen.h"
+
 namespace extensions {
 
 DisplayInfoProviderMac::DisplayInfoProviderMac() {
 }
 
 DisplayInfoProviderMac::~DisplayInfoProviderMac() {
 }
 
 bool DisplayInfoProviderMac::SetInfo(
     const std::string& display_id,
     const core_api::system_display::DisplayProperties& info,
     std::string* error) {
   *error = "Not implemented";
   return false;
 }
 
 void DisplayInfoProviderMac::UpdateDisplayUnitInfoForPlatform(
     const gfx::Display& display,
     extensions::core_api::system_display::DisplayUnitInfo* unit) {
   NOTIMPLEMENTED();
 }
 
 gfx::Screen* DisplayInfoProviderMac::GetActiveScreen() {
-  return NULL;
+  return gfx::Screen::GetNativeScreen();
 }
 
 // static
 DisplayInfoProvider* DisplayInfoProvider::Create() {
   return new DisplayInfoProviderMac();
 }
 
 }  // namespace extensions