Adds a --streaming-compile flag to d8 so that we can test the streaming/
background compilation (e.g. for performance testing).

This differs from the --stress-background-compile flag in two main ways:

    1) It's not a stress test, so it doesn't run a main-thread compile
       for verification, and
    2) It uses the "proper" API, and (like Chromium) pumps the message
       loop while waiting for compilation to complete, so e.g. GC idle
       tasks can run.

Change-Id: I1ea1badf39d25076d95c8d19f173510da277541f
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/2219937
Commit-Queue: Ulan Degenbaev <ulan@chromium.org>
Reviewed-by: Ulan Degenbaev <ulan@chromium.org>
Auto-Submit: Leszek Swirski <leszeks@chromium.org>
Cr-Commit-Position: refs/heads/master@{#68069}