Add more comparators to Optional.h. BUG=angleproject:1387 Change-Id: If3fc67f99716b2e5e6a8b0ffd139a07a06cdcab8 Reviewed-on: https://chromium-review.googlesource.com/648050 Reviewed-by: Jamie Madill <jmadill@chromium.org> Commit-Queue: Jamie Madill <jmadill@chromium.org>