Implements a display scale tray setting behind a chrome switch

ash/system/display_scale/tray_scale.cc

Index: ash/system/display_scale/tray_scale.cc
diff --git a/ash/system/display_scale/tray_scale.cc b/ash/system/display_scale/tray_scale.cc
new file mode 100644
index 0000000000000000000000000000000000000000..3e671898ca65325806319a64030077d15db47119
--- /dev/null
+++ b/ash/system/display_scale/tray_scale.cc
@@ -0,0 +1,59 @@
+// Copyright 2017 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 "ash/system/display_scale/tray_scale.h"
+
+#include "ash/ash_switches.h"
+#include "ash/resources/vector_icons/vector_icons.h"
+#include "ash/root_window_controller.h"
+#include "ash/shell_port.h"
+#include "ash/system/display_scale/scale_detailed_view.h"
+#include "ash/system/display_scale/scale_view.h"
+#include "ash/system/tray/system_tray.h"
+#include "ash/system/tray/tray_constants.h"
+#include "base/command_line.h"
+#include "ui/views/view.h"
+
+namespace ash {
+namespace {
+
+bool IsDisplayScaleTrayEnabled() {
+  return base::CommandLine::ForCurrentProcess()->HasSwitch(
+      switches::kAshEnableScaleSettingsTray);
+}
+
+}  // namespace
+
+TrayScale::TrayScale(SystemTray* system_tray)
+    : SystemTrayItem(system_tray, UMA_NOT_RECORDED) {}
+
+TrayScale::~TrayScale() {}
+
+views::View* TrayScale::CreateDefaultView(LoginStatus status) {
+  if (!IsDisplayScaleTrayEnabled())
+    return nullptr;
+  scale_view_ = new tray::ScaleView(this, true);
+  return scale_view_;
+}
+
+views::View* TrayScale::CreateDetailedView(LoginStatus status) {
+  if (!IsDisplayScaleTrayEnabled())
+    return nullptr;
+  scale_detail_view_ = new tray::ScaleDetailedView(this);
+  return scale_detail_view_;
+}
+
+void TrayScale::DestroyDefaultView() {
+  scale_view_ = nullptr;
+}
+
+void TrayScale::DestroyDetailedView() {
+  scale_detail_view_ = nullptr;
+}
+
+bool TrayScale::ShouldShowShelf() const {
+  return false;
+}
+
+}  // namespace ash