Add GetProcAddress to OpenGL control interface

mojo/public/c/gpu/MGL/mgl.h

Index: mojo/public/c/gpu/MGL/mgl.h
diff --git a/mojo/public/c/gpu/MGL/mgl.h b/mojo/public/c/gpu/MGL/mgl.h
index 2633976ddbdcc82f3cc8147df3e14cd8e036320f..1b5a9a4b9ba859bb9d1461fb0bafd52abfc0305a 100644
--- a/mojo/public/c/gpu/MGL/mgl.h
+++ b/mojo/public/c/gpu/MGL/mgl.h
@@ -55,6 +55,13 @@ void MGLMakeCurrent(MGLContext 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