This moves the initialization of the {Code::stub_key} field into the
allocator for {Code} objects, essentially making the field in question
immutable after allocation.

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

Change-Id: I8ba2ffeea792d0d566995c08e3572ae63a7c1e94
Reviewed-on: https://chromium-review.googlesource.com/739141
Commit-Queue: Michael Starzinger <mstarzinger@chromium.org>
Reviewed-by: Toon Verwaest <verwaest@chromium.org>
Cr-Commit-Position: refs/heads/master@{#48971}