To deserialize builtins individually, we need to preserve their starting
offsets within the serialized data.

Bug: v8:6624
Cq-Include-Trybots: master.tryserver.chromium.linux:linux_chromium_rel_ng
Change-Id: I48a48330aeb63de2c8cfcbea6fb94e1b2917495c
Reviewed-on: https://chromium-review.googlesource.com/637774Reviewed-by: Yang Guo <yangguo@chromium.org>
Commit-Queue: Jakob Gruber <jgruber@chromium.org>
Cr-Commit-Position: refs/heads/master@{#47708}