The irregexp compiler expects RegExpCharacterClass instances to
contain at least one range. This preserves that invariant when parsing
the negated \P{Any} unicode property.

Bug: chromium:793588
Change-Id: If71fdce014a7e64d8af559084ee88108303d694b
Reviewed-on: https://chromium-review.googlesource.com/827010Reviewed-by: Mathias Bynens <mathias@chromium.org>
Reviewed-by: Yang Guo <yangguo@chromium.org>
Reviewed-by: Erik Corry <erikcorry@chromium.org>
Commit-Queue: Jakob Gruber <jgruber@chromium.org>
Cr-Commit-Position: refs/heads/master@{#50146}