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: I25b23b3bf64502f84c28ce688ad2997c9665a069
Reviewed-on: https://chromium-review.googlesource.com/458003Reviewed-by: Daniel Clifford <danno@chromium.org>
Reviewed-by: Marja Hölttä <marja@chromium.org>
Reviewed-by: Daniel Vogelheim <vogelheim@chromium.org>
Commit-Queue: Wiktor Garbacz <wiktorg@google.com>
Cr-Commit-Position: refs/heads/master@{#44140}