This CL extracts some functionality out of the {PrepareAndStartCompile}
step, in order to reuse that from the {AsyncStreamingProcessor}. We
currently schedule a {PrepareAndStartCompile} task to get the same
effect, and rely on the internal implementation to do the right thing.

R=ahaas@chromium.org

Bug: v8:8238
Change-Id: I43135fe488a5f72c09307ac955381c69b7987ec1
Reviewed-on: https://chromium-review.googlesource.com/c/1297321Reviewed-by: Andreas Haas <ahaas@chromium.org>
Commit-Queue: Clemens Hammacher <clemensh@chromium.org>
Cr-Commit-Position: refs/heads/master@{#56983}