This a step towards using CompilerDispatcher in parallel
parsing.

BUG=v8:6093

Change-Id: I7c0dc0a5f5b25652a8aa98c94b1e069c1f081a5b
Reviewed-on: https://chromium-review.googlesource.com/473106Reviewed-by: Jochen Eisinger <jochen@chromium.org>
Commit-Queue: Wiktor Garbacz <wiktorg@google.com>
Cr-Commit-Position: refs/heads/master@{#44570}