Implement --ozone-platform flag to select OzonePlatform implementation at runtime

ui/base/ozone/platform/dri/ozone_platform_dri.h

 // Copyright (c) 2013 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_OZONE_DRM_OZONE_PLATFORM_DRM_H_
 #define UI_OZONE_DRM_OZONE_PLATFORM_DRM_H_
 
 #include "ui/base/ozone/ozone_platform.h"
 #include "ui/events/ozone/evdev/event_factory.h"
 #include "ui/gfx/ozone/impl/software_surface_factory_ozone.h"
 
 namespace ui {
 
 class OzonePlatformDri : public OzonePlatform {
  public:
   OzonePlatformDri();
   virtual ~OzonePlatformDri();
 
   virtual gfx::SurfaceFactoryOzone* GetSurfaceFactoryOzone() OVERRIDE;
   virtual ui::EventFactoryOzone* GetEventFactoryOzone() OVERRIDE;
 
  private:
   gfx::SoftwareSurfaceFactoryOzone surface_factory_ozone_;
   ui::EventFactoryEvdev event_factory_ozone_;
 
   DISALLOW_COPY_AND_ASSIGN(OzonePlatformDri);
 };
 
+// Constructor hook for use in ozone_platform_list.cc
+OzonePlatform* CreateOzonePlatformDri();
+
 }  // namespace ui
 
 #endif  // UI_OZONE_OZONE_PLATFORM_H_