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
TBR=marja@chromium.org,mstarzinger@chromium.org

Cq-Include-Trybots: master.tryserver.chromium.linux:linux_chromium_rel_ng
Change-Id: Icf90ac7211298d3555515dafc7c3245618ec1304
Reviewed-on: https://chromium-review.googlesource.com/764048
Commit-Queue: Ross McIlroy <rmcilroy@chromium.org>
Reviewed-by: Ross McIlroy <rmcilroy@chromium.org>
Cr-Commit-Position: refs/heads/master@{#49364}