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: I977eec8faecc8a112506f219994b4e4ec388b180
Reviewed-on: https://chromium-review.googlesource.com/458004
Commit-Queue: Wiktor Garbacz <wiktorg@google.com>
Reviewed-by: Ulan Degenbaev <ulan@chromium.org>
Reviewed-by: Daniel Vogelheim <vogelheim@chromium.org>
Reviewed-by: Marja Hölttä <marja@chromium.org>
Cr-Commit-Position: refs/heads/master@{#44160}