[pointer-lock] pointerLockElement should be nullable in the IDL

[Boris Zbarsky]

The IDL at 
http://dvcs.w3.org/hg/pointerlock/raw-file/default/index.html#extensions-to-the-document-interface 
says:

    readonly attribute Element pointerLockElement;
but the attribute can return null, so the type should be "Element?".

-Boris