Add ifdef guards to prepare for WebLayerTreeView interface change

ui/compositor/compositor.h

 // Copyright (c) 2012 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_COMPOSITOR_COMPOSITOR_H_
 #define UI_COMPOSITOR_COMPOSITOR_H_
 
 #include "base/hash_tables.h"
 #include "base/memory/ref_counted.h"
 #include "base/memory/scoped_ptr.h"
@@ skipping 223 matching lines @@
   CompositorDelegate* delegate_;
   gfx::Size size_;
 
   // The root of the Layer tree drawn by this compositor.
   Layer* root_layer_;
 
   ObserverList<CompositorObserver> observer_list_;
 
   gfx::AcceleratedWidget widget_;
   scoped_ptr<WebKit::WebLayer> root_web_layer_;
+#if defined(WEBLAYERTREEVIEW_IS_PURE_VIRTUAL)
+  scoped_ptr<WebKit::WebLayerTreeView> host_;
+#else
   WebKit::WebLayerTreeView host_;
+#endif
 
   // This is set to true when the swap buffers has been posted and we're waiting
   // for completion.
   bool swap_posted_;
 
   // The device scale factor of the monitor that this compositor is compositing
   // layers on.
   float device_scale_factor_;
 
   int last_started_frame_;
   int last_ended_frame_;
 
   bool disable_schedule_composite_;
 
   DISALLOW_COPY_AND_ASSIGN(Compositor);
 };
 
 }  // namespace ui
 
 #endif  // UI_COMPOSITOR_COMPOSITOR_H_