function removeGlobalKeydownHook(hook) {
        var index = _globalKeydownHooks.indexOf(hook);
        if (index !== -1) {
            _globalKeydownHooks.splice(index, 1);
        }
    }
Removes a global keydown hook added by `addGlobalKeydownHook`.
Does not need to be the most recently added hook.

@param {function(Event): boolean} hook The global hook to remove.