Make GL calls go through subclassable class.

ui/gl/gl_surface_glx.cc

Index: ui/gl/gl_surface_glx.cc
diff --git a/ui/gl/gl_surface_glx.cc b/ui/gl/gl_surface_glx.cc
index 6e1675498ba3bfac0af052f9c28b0043ce37913c..f810114b1a7605704bf81db92fdca27f99b87598 100644
--- a/ui/gl/gl_surface_glx.cc
+++ b/ui/gl/gl_surface_glx.cc
@@ -122,7 +122,7 @@ bool NativeViewGLSurfaceGLX::Resize(const gfx::Size& size) {
   // On Intel drivers, the frame buffer won't be resize until the next swap. If
   // we only do PostSubBuffer, then we're stuck in the old size. Force a swap
   // now.
-  if (gfx::g_GLX_MESA_copy_sub_buffer && size_ != size)
+  if (gfx::g_driver_glx.ext.b_GLX_MESA_copy_sub_buffer && size_ != size)
     SwapBuffers();
   size_ = size;
   return true;
@@ -149,7 +149,7 @@ void* NativeViewGLSurfaceGLX::GetHandle() {
 
 std::string NativeViewGLSurfaceGLX::GetExtensions() {
   std::string extensions = GLSurface::GetExtensions();
-  if (g_GLX_MESA_copy_sub_buffer) {
+  if (gfx::g_driver_glx.ext.b_GLX_MESA_copy_sub_buffer) {
     extensions += extensions.empty() ? "" : " ";
     extensions += "GL_CHROMIUM_post_sub_buffer";
   }
@@ -219,7 +219,7 @@ void* NativeViewGLSurfaceGLX::GetConfig() {
 
 bool NativeViewGLSurfaceGLX::PostSubBuffer(
     int x, int y, int width, int height) {
-  DCHECK(g_GLX_MESA_copy_sub_buffer);
+  DCHECK(gfx::g_driver_glx.ext.b_GLX_MESA_copy_sub_buffer);
   glXCopySubBufferMESA(g_display, window_, x, y, width, height);
   return true;
 }