Add GetProcAddress to OpenGL control interface

mojo/public/platform/native/mgl_thunks.c

Index: mojo/public/platform/native/mgl_thunks.c
diff --git a/mojo/public/platform/native/mgl_thunks.c b/mojo/public/platform/native/mgl_thunks.c
index 2e8e9d84c37e3aa3c97f73d03f7616ffccfb87fe..9b74f0c210ff8a73b9b762e4f8b395f52c3a034d 100644
--- a/mojo/public/platform/native/mgl_thunks.c
+++ b/mojo/public/platform/native/mgl_thunks.c
@@ -43,3 +43,8 @@ THUNK_EXPORT size_t MojoSetMGLThunks(
     g_thunks = *mgl_thunks;
   return sizeof(g_thunks);
 }
+
+MGLMustCastToProperFunctionPointerType MGLGetProcAddress(const char* name) {
+  assert(g_thunks.MGLGetProcAddress);
+  return g_thunks.MGLGetProcAddress(name);
+}