Make GL calls go through subclassable class.

ui/gl/gl_context_glx.cc

Index: ui/gl/gl_context_glx.cc
diff --git a/ui/gl/gl_context_glx.cc b/ui/gl/gl_context_glx.cc
index dc8226b4b50eb224baba5eb5603a150f18eb79a1..3ab750b9ed923f430522147f1d408c978872f15c 100644
--- a/ui/gl/gl_context_glx.cc
+++ b/ui/gl/gl_context_glx.cc
@@ -219,12 +219,14 @@ void* GLContextGLX::GetHandle() {
 
 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)