Moves ownership of the parsing Zone to ParseInfo with a shared_ptr. This is
in preperation for enabling background compilation jobs for inner functions
share the AST in the outer-function's parse zone memory (read-only), with the
and zone being released when all compilation jobs have completed.

BUG=v8:5203,v8:5215

Review-Url: https://codereview.chromium.org/2632123006
Cr-Commit-Position: refs/heads/master@{#42993}