Recent changes in SerializerReferences (see https://crrev.com/c/1113930)
opened up these minor improvements to the builtin serializer allocator,
since we now have dedicated BuiltinReferences.

R=yangguo@chromium.org

Bug: v8:7887
Change-Id: Ie76a52a88ef4075c63c5eb2eaa81382c52325688
Reviewed-on: https://chromium-review.googlesource.com/1125844Reviewed-by: Yang Guo <yangguo@chromium.org>
Commit-Queue: Jakob Gruber <jgruber@chromium.org>
Cr-Commit-Position: refs/heads/master@{#54207}