Speculative revert in the hopes of fixing serializer crashes seen in canary.

This reverts commit c1669450, as well as
followup change "Do not look for existing shared function info when compiling a new script."
(commit 7c43967b).

BUG=chromium:503552,v8:4132
TBR=yangguo@chromium.org
LOG=n

Review URL: https://codereview.chromium.org/1207583002

Cr-Commit-Position: refs/heads/master@{#29241}