Treat keyCode 10 as "return" in JX.Event

Summary: Fixes T2753. Return is sent as either 10 or 13, depending on browser and OS. See, e.g. this Chromium issue.

Test Plan: @cystorm confirmed this patch fixes the issue on his system.

Reviewers: btrahan, codeblock

Reviewed By: codeblock

CC: cystorm, aran

Maniphest Tasks: T2753

Differential Revision: https://secure.phabricator.com/D5381