Add mojom::DisplayController.

services/ui/display/platform_screen.cc

+// Copyright 2016 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 "services/ui/display/platform_screen.h"
+
+#include "base/logging.h"
+
+namespace display {
+
+// static
+PlatformScreen* PlatformScreen::instance_ = nullptr;
+
+PlatformScreen::PlatformScreen() {
+  DCHECK(!instance_);
+  instance_ = this;
+}
+
+PlatformScreen::~PlatformScreen() {
+  DCHECK_EQ(instance_, this);
+  instance_ = nullptr;
+}
+
+// static
+PlatformScreen* PlatformScreen::GetInstance() {
+  DCHECK(instance_);
+  return instance_;
+}
+
+}  // namespace display