ui: Cleanup class/struct forward declarations

ui/gl/test/gl_surface_test_support.h

 // Copyright 2015 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_TEST_GL_SURFACE_TEST_SUPPORT_H_
 #define UI_GL_TEST_GL_SURFACE_TEST_SUPPORT_H_
 
 #include "ui/gl/gl_implementation.h"
 
 namespace gl {
 
-class GLContext;
-
 class GLSurfaceTestSupport {
  public:
   static void InitializeOneOff();
   static void InitializeOneOffImplementation(GLImplementation impl,
                                              bool fallback_to_osmesa);
   static void InitializeOneOffWithMockBindings();
 };
 
 }  // namespace gl
 
 #endif  // UI_GL_TEST_GL_SURFACE_TEST_SUPPORT_H_