Track if a non-preprocessor token has been seen and validate #extension with it. Reland: Only report a warning instead of an error. BUG=angleproject:989 BUG=483252 Change-Id: Ibf9adbf423cd9dee20ec45b8d2ea42bcfd9311be Reviewed-on: https://chromium-review.googlesource.com/269205 Reviewed-by: Jamie Madill <jmadill@chromium.org> Reviewed-by: Zhenyao Mo <zmo@chromium.org> Tested-by: Geoff Lang <geofflang@chromium.org>