Refactor screen/monitor so that gfx::Screen returns monitor object.

ui/aura/monitor.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/aura/monitor.h"
-
-namespace aura {
-
-Monitor::Monitor() : device_scale_factor_(1.0f) {
-}
-
-Monitor::~Monitor() {
-}
-
-gfx::Rect Monitor::GetWorkAreaBounds() const {
-  // TODO(oshima): For m19, work area/monitor bounds has (0,0) origin
-  // because it's simpler and enough. Fix this when real multi monitor
-  // support is implemented.
-  gfx::Rect bounds(bounds_.size());
-  bounds.Inset(work_area_insets_);
-  return bounds;
-}
-
-}  // namespace aura