aura: Have aura and aura_demo build and run without gtk on linux.

ui/base/x/active_window_watcher_x.h

 // Copyright (c) 2011 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.
 
 #ifndef UI_BASE_X_ACTIVE_WINDOW_WATCHER_X_H_
 #define UI_BASE_X_ACTIVE_WINDOW_WATCHER_X_H_
 #pragma once
 
-#include <gdk/gdk.h>
-
 #include "base/basictypes.h"
 #include "base/memory/singleton.h"
 #include "base/observer_list.h"
 #include "ui/base/gtk/gtk_signal.h"
 #include "ui/base/ui_export.h"
 
+#if defined(TOOLKIT_USES_GTK)
+#include <gdk/gdk.h>
+#endif
+
 namespace ui {
 
 // This is a helper class that is used to keep track of which window the X
 // window manager thinks is active. Add an Observer to listener for changes to
 // the active window.
 class UI_EXPORT ActiveWindowWatcherX {
  public:
   class Observer {
    public:
+#if defined(TOOLKIT_USES_GTK)
     // |active_window| will be NULL if the active window isn't one of Chrome's.
     virtual void ActiveWindowChanged(GdkWindow* active_window) = 0;
+#endif
 
    protected:
     virtual ~Observer() {}
   };
 
   static ActiveWindowWatcherX* GetInstance();
 
   static void AddObserver(Observer* observer);
   static void RemoveObserver(Observer* observer);
 
   // Checks if the WM supports the active window property. Note that the return
   // value can change, especially during system startup.
   static bool WMSupportsActivation();
 
  private:
   friend struct DefaultSingletonTraits<ActiveWindowWatcherX>;
 
   ActiveWindowWatcherX();
   virtual ~ActiveWindowWatcherX();
 
   void Init();
 
   // Sends a notification out through the NotificationService that the active
   // window has changed.
   void NotifyActiveWindowChanged();
 
+#if defined(TOOLKIT_USES_GTK)
   // Callback for PropertyChange XEvents.
   CHROMEG_CALLBACK_1(ActiveWindowWatcherX, GdkFilterReturn,
                      OnWindowXEvent, GdkXEvent*, GdkEvent*);
+#endif
 
   ObserverList<Observer> observers_;
 
   DISALLOW_COPY_AND_ASSIGN(ActiveWindowWatcherX);
 };
 
 }  // namespace ui
 
 #endif  // UI_BASE_X_ACTIVE_WINDOW_WATCHER_X_H_