A step towards removing isolate from ParseInfo.
Removing isolate from ParseInfo will make it easier to create and
execute parse tasks on background threads.

BUG=v8:6093

Change-Id: If5e7ba2f76f9f8422c26f47c9f0eb5b197114b25
Reviewed-on: https://chromium-review.googlesource.com/458000Reviewed-by: Daniel Vogelheim <vogelheim@chromium.org>
Reviewed-by: Marja Hölttä <marja@chromium.org>
Commit-Queue: Wiktor Garbacz <wiktorg@google.com>
Cr-Commit-Position: refs/heads/master@{#44089}