Define 'register' as an empty string if we're in a modern version
of C++.

BUG=255186
BUG=angleproject:463

Change-Id: Ied044fb87a9b05c91cb419c54295c39f0f0ab776
Reviewed-on: https://chromium-review.googlesource.com/268512Reviewed-by: Geoff Lang <geofflang@chromium.org>
Tested-by: Jamie Madill <jmadill@chromium.org>
Reviewed-by: Zhenyao Mo <zmo@chromium.org>