Makes ash/wm/common a library

ash/wm/window_state_aura.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 "ash/wm/window_state_aura.h"
 
 #include "ash/wm/aura/wm_window_aura.h"
 #include "ash/wm/common/window_state.h"
 #include "ash/wm/window_properties.h"
 #include "ash/wm/window_util.h"
 #include "ui/aura/window.h"
 
 namespace ash {
 namespace wm {
+namespace {
+
+// This classes is used so that the WindowState constructor can be made
+// protected. GetWindowState() is the only place that should be creating
+// WindowState.
+class WindowStateAura : public wm::WindowState {
+ public:
+  explicit WindowStateAura(WmWindow* window) : wm::WindowState(window) {}
+  ~WindowStateAura() override {}
+
+ private:
+  DISALLOW_COPY_AND_ASSIGN(WindowStateAura);
+};
+
+}  // namespace
 
 WindowState* GetActiveWindowState() {
   aura::Window* active = GetActiveWindow();
   return active ? GetWindowState(active) : nullptr;
 }
 
 WindowState* GetWindowState(aura::Window* window) {
   if (!window)
     return nullptr;
   WindowState* settings = window->GetProperty(kWindowStateKey);
   if (!settings) {
-    settings = new WindowState(WmWindowAura::Get(window));
+    settings = new WindowStateAura(WmWindowAura::Get(window));
     window->SetProperty(kWindowStateKey, settings);
   }
   return settings;
 }
 
 const WindowState* GetWindowState(const aura::Window* window) {
   return GetWindowState(const_cast<aura::Window*>(window));
 }
 
 }  // namespace wm
 }  // namespace ash