After allocation of an object, we need to initialize it to make it safe
for the GC to see it. For complex objects like SharedFunctionInfo, this
initialization code is long and requires understanding of the object. So,
it makes sense for the initialization to live in the SharedFunctionInfo
code itself (as an Init method) rather than in the factory.

Aside from being a neat cleanup, this will allow us to share this
initialization logic between different allocation methods, as part of the
off-thread allocation project:
https://docs.google.com/document/d/1-_96kok0AcavkbcdqqZvpqt_2q-_XWAsAJwbRXlfwCo/

Bug: chromium:1011762
Change-Id: Ie276eb711423272f85abfeb3d88df1826a77b984
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/1872402
Commit-Queue: Leszek Swirski <leszeks@chromium.org>
Reviewed-by: Toon Verwaest <verwaest@chromium.org>
Reviewed-by: Ulan Degenbaev <ulan@chromium.org>
Cr-Commit-Position: refs/heads/master@{#64926}