Add GetProcAddress to OpenGL control interface

mojo/public/platform/native/mgl_thunks.c

 // Copyright 2015 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.
 
 #include "mojo/public/platform/native/mgl_thunks.h"
 
 #include <assert.h>
 
 #include "mojo/public/platform/native/thunk_export.h"
 
@@ skipping 25 matching lines @@
   assert(g_thunks.MGLGetCurrentContext);
   return g_thunks.MGLGetCurrentContext();
 }
 
 THUNK_EXPORT size_t MojoSetMGLThunks(
     const struct MGLThunks* mgl_thunks) {
   if (mgl_thunks->size >= sizeof(g_thunks))
     g_thunks = *mgl_thunks;
   return sizeof(g_thunks);
 }
+
+MGLMustCastToProperFunctionPointerType MGLGetProcAddress(const char* name) {
+  assert(g_thunks.MGLGetProcAddress);
+  return g_thunks.MGLGetProcAddress(name);
+}