Revert of [ast] Simplify FetchFreeVariables. (patchset #1 id:1 of https://codereview.chromium.org/2491373004/ )

Reason for revert:
It's probably needed after all but we're lacking tests.

Original issue's description:
> [ast] Simplify FetchFreeVariables.
>
> This CL removes the ParseInfo argument from FetchFreeVariables, since it seems
> to have become unnecessary.
>
> R=verwaest@chromium.org
> BUG=

TBR=verwaest@chromium.org
# Not skipping CQ checks because original CL landed more than 1 days ago.
BUG=

Review-Url: https://codereview.chromium.org/2495293002
Cr-Commit-Position: refs/heads/master@{#40964}