In order to refactor ownership between objects in wasm compilation, the
compilation (executed by background tasks) is split in three stages:
getting a compilation unit (while holding a mutex), executing the work
(without any mutex and without keeping the NativeModule alive), and
submitting the work (with a mutex again).

This CL prepares this design by splitting compilation from submission.
Both steps are still executed right after each other. This will be
changed in a follow-up CL.

R=titzer@chromium.org
CC=mstarzinger@chromium.org

Bug: v8:8689
Change-Id: I2f92aee8e2f2d45470d8c63314ed026341630902
Reviewed-on: https://chromium-review.googlesource.com/c/1414920Reviewed-by: Ben Titzer <titzer@chromium.org>
Commit-Queue: Clemens Hammacher <clemensh@chromium.org>
Cr-Commit-Position: refs/heads/master@{#58929}