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: Ife3e7759cdef6bc0f41cae3c58c307682b608279 Reviewed-on: https://chromium-review.googlesource.com/269404 Reviewed-by: Jamie Madill <jmadill@chromium.org> Tested-by: Geoff Lang <geofflang@chromium.org>