Add GetProcAddress to OpenGL control interface

mojo/public/platform/native/mgl_thunks.h

 // 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.
 
 #ifndef MOJO_PUBLIC_PLATFORM_NATIVE_MGL_THUNKS_H_
 #define MOJO_PUBLIC_PLATFORM_NATIVE_MGL_THUNKS_H_
 
 #include <stddef.h>
 
 #include "mojo/public/c/gpu/MGL/mgl.h"
 
 // Structure used to bind the interface which manipulates MGL contexts to a
 // DSO to theose of the embedder.
 //
 // This is the ABI between the embedder and the DSO. It can only have new
 // functions added to the end. No other changes are supported.
 #pragma pack(push, 8)
 struct MGLThunks {
   size_t size;  // Should be set to sizeof(MGLThunks).
 
   MGLContext (*MGLCreateContext)(MGLOpenGLAPIVersion version,
                                  MojoHandle command_buffer_handle,
                                  MGLContext share_group,
                                  MGLContextLostCallback lost_callback,
                                  void* lost_callback_closure,
                                  const struct MojoAsyncWaiter* async_waiter);
   void (*MGLDestroyContext)(MGLContext context);
   void (*MGLMakeCurrent)(MGLContext context);
   MGLContext (*MGLGetCurrentContext)(void);
+  MGLMustCastToProperFunctionPointerType (*MGLGetProcAddress)(const char* name);
 };
 #pragma pack(pop)
 
 // Intended to be called from the embedder. Returns an object initialized to
 // contain pointers to each of the embedder's MGLThunks functions.
 inline struct MGLThunks MojoMakeMGLThunks() {
   struct MGLThunks mgl_thunks = {
       sizeof(struct MGLThunks),
       MGLCreateContext,
       MGLDestroyContext,
       MGLMakeCurrent,
       MGLGetCurrentContext,
+      MGLGetProcAddress,
   };
 
   return mgl_thunks;
 }
 
 // Use this type for the function found by dynamically discovering it in
 // a DSO linked with mojo_system. For example:
 // MojoSetMGLThunksFn mojo_set_gles2_thunks_fn =
 //     reinterpret_cast<MojoSetMGLThunksFn>(
 //         app_library.GetFunctionPointer("MojoSetMGLThunks"));
 // The expected size of |mgl_thunks| is returned.
 // The contents of |mgl_thunks| are copied.
 typedef size_t (*MojoSetMGLThunksFn)(const struct MGLThunks* mgl_thunks);
 
 #endif  // MOJO_PUBLIC_PLATFORM_NATIVE_MGL_THUNKS_H_