Add a few missing overrides found by a new clang warning.

ui/gl/gl_surface_glx.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_GL_GL_SURFACE_GLX_H_
 #define UI_GL_GL_SURFACE_GLX_H_
 
 #include <string>
 
 #include "base/compiler_specific.h"
@@ skipping 21 matching lines @@
   static bool HasGLXExtension(const char* name);
   static bool IsCreateContextSupported();
   static bool IsCreateContextRobustnessSupported();
   static bool IsTextureFromPixmapSupported();
   static bool IsOMLSyncControlSupported();
 
   void* GetDisplay() override;
 
   // Get the FB config that the surface was created with or NULL if it is not
   // a GLX drawable.
-  virtual void* GetConfig() = 0;
+  virtual void* GetConfig() override = 0;
 
  protected:
   ~GLSurfaceGLX() override;
 
  private:
   DISALLOW_COPY_AND_ASSIGN(GLSurfaceGLX);
 };
 
 // A surface used to render to a view.
 class GL_EXPORT NativeViewGLSurfaceGLX : public GLSurfaceGLX,
@@ skipping 60 matching lines @@
   gfx::Size size_;
   void* config_;
   XID pbuffer_;
 
   DISALLOW_COPY_AND_ASSIGN(PbufferGLSurfaceGLX);
 };
 
 }  // namespace gfx
 
 #endif  // UI_GL_GL_SURFACE_GLX_H_