Encapsulates special reservation / allocation behavior for builtin
serialization.

This allows us to remove special logic around kNextChunk in builtin
deserialization (since we don't generate that bytecode anymore for
builtins).

Bug: v8:6624
Change-Id: Ice7673006cee53b9d11cdfb7f84d4175221c7984
Reviewed-on: https://chromium-review.googlesource.com/720357Reviewed-by: Peter Marshall <petermarshall@chromium.org>
Commit-Queue: Jakob Gruber <jgruber@chromium.org>
Cr-Commit-Position: refs/heads/master@{#48725}