Make GL calls go through subclassable class.

ui/gl/gl_context_wgl.cc

Index: ui/gl/gl_context_wgl.cc
diff --git a/ui/gl/gl_context_wgl.cc b/ui/gl/gl_context_wgl.cc
index e6b8307621650248ec42ee961a785abf26bc9a9b..c1366414d77de1a74f68ea0fa681990aed1b299b 100644
--- a/ui/gl/gl_context_wgl.cc
+++ b/ui/gl/gl_context_wgl.cc
@@ -25,9 +25,9 @@ GLContextWGL::~GLContextWGL() {
 
 std::string GLContextWGL::GetExtensions() {
   const char* extensions = NULL;
-  if (wglGetExtensionsStringARB)
+  if (g_driver_wgl.fn.wglGetExtensionsStringARBFn)
     extensions = wglGetExtensionsStringARB(GLSurfaceWGL::GetDisplayDC());
-  else if (wglGetExtensionsStringEXT)
+  else if (g_driver_wgl.fn.wglGetExtensionsStringEXTFn)
     extensions = wglGetExtensionsStringEXT();
 
   if (extensions)
@@ -129,7 +129,7 @@ void* GLContextWGL::GetHandle() {
 
 void GLContextWGL::SetSwapInterval(int interval) {
   DCHECK(IsCurrent(NULL));
-  if (gfx::g_WGL_EXT_swap_control) {
+  if (gfx::g_driver_wgl.ext.b_WGL_EXT_swap_control) {
     wglSwapIntervalEXT(interval);
   } else {
       LOG(WARNING) <<