This removes the builtins area from the startup snapshot. It's now
completely contained in the separate builtins blob area.

Bug: v8:6624
Change-Id: Id3c43a177c7e1ed418eec59cf620fa461eb6df81
Reviewed-on: https://chromium-review.googlesource.com/715759
Commit-Queue: Jakob Gruber <jgruber@chromium.org>
Reviewed-by: Yang Guo <yangguo@chromium.org>
Cr-Commit-Position: refs/heads/master@{#48636}