Add GetProcAddress to OpenGL control interface

mojo/gles2/mgl_impl.cc

 // 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.
 
 // This file implements the MGL and MGL onscreen entry points exposed to the
 // Mojo application by the shell.
 
 #include "mojo/gles2/control_thunks_impl.h"
 #include "mojo/public/c/gpu/MGL/mgl.h"
 #include "mojo/public/c/gpu/MGL/mgl_onscreen.h"
@@ skipping 31 matching lines @@
 }
 
 void MGLResizeSurface(uint32_t width, uint32_t height) {
   return gles2::ControlThunksImpl::Get()->ResizeSurface(width, height);
 }
 
 void MGLSwapBuffers() {
   return gles2::ControlThunksImpl::Get()->SwapBuffers();
 }
 
+MGLFunctionPointerType MGLGetProcAddress(const char* procname) {
+  return gles2::ControlThunksImpl::Get()->GetProcAddress(procname);
+}
+
 }  // extern "C"