Adds support for compiling top-level code on a background thread behind a flag.
When the flag is enabled, any background-parsing-task will perform compilation
as well as parsing.

BUG=v8:5203

Cq-Include-Trybots: master.tryserver.chromium.linux:linux_chromium_rel_ng
Change-Id: I88ab05c97cd6aea8d6be26e27d8da327f2c9c3a8
Reviewed-on: https://chromium-review.googlesource.com/741716
Commit-Queue: Ross McIlroy <rmcilroy@chromium.org>
Reviewed-by: Michael Starzinger <mstarzinger@chromium.org>
Reviewed-by: Marja Hölttä <marja@chromium.org>
Cr-Commit-Position: refs/heads/master@{#49103}