Make GL calls go through subclassable class.

ui/gl/gl_context_glx.cc

 // 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.
 
 extern "C" {
 #include <X11/Xlib.h>
 }
 
 #include "ui/gl/gl_context_glx.h"
 
@@ skipping 201 matching lines @@
 
   return true;
 }
 
 void* GLContextGLX::GetHandle() {
   return context_;
 }
 
 void GLContextGLX::SetSwapInterval(int interval) {
   DCHECK(IsCurrent(NULL));
-  if (HasExtension("GLX_EXT_swap_control") && glXSwapIntervalEXT) {
+  if (HasExtension("GLX_EXT_swap_control") &&
+      g_driver_glx.fn.glXSwapIntervalEXTFn) {
     glXSwapIntervalEXT(
         display_,
         glXGetCurrentDrawable(),
         interval);
-  } else if (HasExtension("GLX_MESA_swap_control") && glXSwapIntervalMESA) {
+  } else if (HasExtension("GLX_MESA_swap_control") &&
+             g_driver_glx.fn.glXSwapIntervalMESAFn) {
     glXSwapIntervalMESA(interval);
   } else {
     if(interval == 0)
       LOG(WARNING) <<
           "Could not disable vsync: driver does not "
           "support GLX_EXT_swap_control";
   }
 }
 
 std::string GLContextGLX::GetExtensions() {
@@ skipping 20 matching lines @@
 
 bool GLContextGLX::WasAllocatedUsingRobustnessExtension() {
   return GLSurfaceGLX::IsCreateContextRobustnessSupported();
 }
 
 GLContextGLX::~GLContextGLX() {
   Destroy();
 }
 
 }  // namespace gfx