Add GetProcAddress to OpenGL control interface

mojo/public/platform/native/mgl_thunks.h

Index: mojo/public/platform/native/mgl_thunks.h
diff --git a/mojo/public/platform/native/mgl_thunks.h b/mojo/public/platform/native/mgl_thunks.h
index 8a56474ea03227850cca9797bfe149b266268f7b..03bc18edc33c4d4c6be55ae227c6f3cbe5787661 100644
--- a/mojo/public/platform/native/mgl_thunks.h
+++ b/mojo/public/platform/native/mgl_thunks.h
@@ -27,6 +27,7 @@ struct MGLThunks {
   void (*MGLDestroyContext)(MGLContext context);
   void (*MGLMakeCurrent)(MGLContext context);
   MGLContext (*MGLGetCurrentContext)(void);
+  void* (*MGLGetProcAddress)(const char* name);
 };
 #pragma pack(pop)
 
@@ -39,6 +40,7 @@ inline struct MGLThunks MojoMakeMGLThunks() {
       MGLDestroyContext,
       MGLMakeCurrent,
       MGLGetCurrentContext,
+      MGLGetProcAddress,
   };
 
   return mgl_thunks;