Add GetProcAddress to OpenGL control interface

mojo/public/c/gpu/MGL/mgl.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.
 
 // Note: This header should be compilable as C.
 
 #ifndef MOJO_PUBLIC_C_GPU_MGL_MGL_H_
 #define MOJO_PUBLIC_C_GPU_MGL_MGL_H_
 
 #include <stdint.h>
@@ skipping 37 matching lines @@
 void MGLDestroyContext(MGLContext context);
 
 // Makes |context| the current MGLContext for the calling thread. Calling with
 // MGL_NO_CONTEXT clears the current context.
 void MGLMakeCurrent(MGLContext context);
 
 // Returns the currently bound context for the calling thread or MGL_NO_CONTEXT
 // if there is none.
 MGLContext MGLGetCurrentContext(void);
 
+// Returns GL function usable in any context that advertise the corresponding
+// extension in their GL_EXTENSIONS string, or null for functions that the
+// implementation does not support. The implementation only advertises GL
+// functions.
+// |name| is the name of the GL function.
+MGLMustCastToProperFunctionPointerType MGLGetProcAddress(const char* name);
+
 #ifdef __cplusplus
 }  // extern "C"
 #endif
 
 #endif  // MOJO_PUBLIC_C_GPU_MGL_MGL_H_