This a step towards using CompilerDispatcher in parallel
parsing.

BUG=v8:6093

Change-Id: Idee84105e342950badb5694fa2a850e05430abaa
Reviewed-on: https://chromium-review.googlesource.com/473246
Commit-Queue: Wiktor Garbacz <wiktorg@google.com>
Reviewed-by: Jochen Eisinger <jochen@chromium.org>
Reviewed-by: Marja Hölttä <marja@chromium.org>
Cr-Commit-Position: refs/heads/master@{#44573}