This is the first step towards lazy-deserializing bytecode handlers.

Bytecode handler code objects are now serialized into the builtins
snapshot area (which, like many other related concepts, has become
somewhat of a misnomer now that it contains both builtins and
handlers).

Handlers are still eagerly-deserialized upon Isolate creation. This will
change in follow-up CLs.

Bug: v8:6624
Cq-Include-Trybots: master.tryserver.chromium.linux:linux_chromium_rel_ng
Change-Id: I7b257f76f5e9e90d5f7b183980bae7bc621171fc
Reviewed-on: https://chromium-review.googlesource.com/738030
Commit-Queue: Jakob Gruber <jgruber@chromium.org>
Reviewed-by: Peter Marshall <petermarshall@chromium.org>
Reviewed-by: Mythri Alle <mythria@chromium.org>
Cr-Commit-Position: refs/heads/master@{#48977}