Remove Glutin dependency for WebGL

Now we have mac support, and since android build is broken, we can
drop glutin from WebGL code.

Went back to upstream
(see: https://github.com/servo/rust-offscreen-rendering-context/pull/1#issuecomment-99234534)

Rebased

components/canvas/lib.rs

@@ -14,7 +14,6 @@ extern crate util;
 extern crate gleam;
 extern crate num;
 extern crate offscreen_gl_context;
-extern crate glutin;
 
 #[macro_use]
 extern crate log;