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

ui/base/x/active_window_watcher_x.h

Index: ui/base/x/active_window_watcher_x.h
diff --git a/ui/base/x/active_window_watcher_x.h b/ui/base/x/active_window_watcher_x.h
index 16c87a35f04163b7f6f3d3d0995dba81e0ddc1f9..a795437da89d7594cf50766ae3ac5d754f52e399 100644
--- a/ui/base/x/active_window_watcher_x.h
+++ b/ui/base/x/active_window_watcher_x.h
@@ -6,14 +6,16 @@
 #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
@@ -23,8 +25,10 @@ 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() {}
@@ -51,9 +55,11 @@ class UI_EXPORT ActiveWindowWatcherX {
   // 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_;