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: I2a3a8a02d791d7ae6acc2243e2f430defc3be5ea
Reviewed-on: https://chromium-review.googlesource.com/458002
Commit-Queue: Wiktor Garbacz <wiktorg@google.com>
Reviewed-by: Daniel Vogelheim <vogelheim@chromium.org>
Reviewed-by: Marja Hölttä <marja@chromium.org>
Cr-Commit-Position: refs/heads/master@{#44097}