Fix tokenizer input of Japanese glyphs

Summary: Fixes T3834. We have some hack code here for Firefox, but I can't reproduce the original Firefox issue in modern Firefox. It's better to break weird copy/paste edge cases than all Japanese input, in any case.

Test Plan: See screenshot.

Reviewers: chad, btrahan

Reviewed By: chad

CC: aran

Maniphest Tasks: T3834

Differential Revision: https://secure.phabricator.com/D7024