cocoa browser: bind cmd-opt-l, not cmd-l, for downloads

A longstanding trap in global_keyboard_shortcuts_mac: if a binding has a
character specified instead of a keycode, the option modifier in the command is
ignored, which makes sense, since option might be involved in typing the
character in the first place. However, it does mean that bindings should never
specify opt == true, which is exactly what I did in CL 2369453003. This CL adds
a DCHECK to ensure nobody else trips over this.

BUG=650528

Review-Url: https://codereview.chromium.org/2373543003
Cr-Commit-Position: refs/heads/master@{#421203}