This makes sure flags on newly allocated {Code} objects are initialized
from within the allocator itself instead of after the object has been
created. It essentially makes these flags immutable.

R=jarin@chromium.org
BUG=v8:6792

Change-Id: I6bef183a25508faf1fec28d347956e766e65aecf
Reviewed-on: https://chromium-review.googlesource.com/737633
Commit-Queue: Michael Starzinger <mstarzinger@chromium.org>
Reviewed-by: Jaroslav Sevcik <jarin@chromium.org>
Cr-Commit-Position: refs/heads/master@{#48933}