Make GL calls go through subclassable class.

ui/gl/gl_surface_wgl.cc

Index: ui/gl/gl_surface_wgl.cc
diff --git a/ui/gl/gl_surface_wgl.cc b/ui/gl/gl_surface_wgl.cc
index a72c825a185268e4555c8b6ec6275d4cec4e7325..7e1c3a7ad1fe9f2ec1cf127ba6eeadfb1f80b0b3 100644
--- a/ui/gl/gl_surface_wgl.cc
+++ b/ui/gl/gl_surface_wgl.cc
@@ -7,6 +7,8 @@
 #include "base/logging.h"
 #include "base/memory/scoped_ptr.h"
 #include "ui/gl/gl_bindings.h"
+#include "ui/gl/gl_gl_api_implementation.h"
+#include "ui/gl/gl_wgl_api_implementation.h"
 
 namespace gfx {
 
@@ -306,7 +308,7 @@ PbufferGLSurfaceWGL::~PbufferGLSurfaceWGL() {
 bool PbufferGLSurfaceWGL::Initialize() {
   DCHECK(!device_context_);
 
-  if (!wglCreatePbufferARB) {
+  if (!gfx::g_driver_wgl.fn.wglCreatePbufferARBFn) {
     LOG(ERROR) << "wglCreatePbufferARB not available.";
     Destroy();
     return false;