Speculative reason for issue 684481.

BUG=chromium:684481
TBR=marja@chromium.org,mstarzinger@chromium.org,ahaas@chromium.org,verwaest@chromium.org,

Original issue's description:
> [Parse] ParseInfo owns the parsing Zone.
>
> 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@{#42562}
> Committed: https://chromium.googlesource.com/v8/v8/+/4b0101d369af121a6daf5e5c731cb939c026d526

Review-Url: https://codereview.chromium.org/2648383005
Cr-Commit-Position: refs/heads/master@{#42633}