This is a refactoring-only CL that packs builtin offsets and lengths
into a metadata struct. The struct should make future modifications to
metadata simpler.

Bug: v8:6666
Change-Id: Iae8070c2aa24d7e803cd75da3dea09e437ef275e
Reviewed-on: https://chromium-review.googlesource.com/1145069
Commit-Queue: Jakob Gruber <jgruber@chromium.org>
Reviewed-by: Sigurd Schneider <sigurds@chromium.org>
Cr-Commit-Position: refs/heads/master@{#54729}