This CL adds an API function that tells the embedder if there is ongoing
background work that will eventually post foreground tasks.

Design doc: https://docs.google.com/document/d/18vaABH1mR35PQr8XPHZySuQYgSjJbWFyAW63LW2m8-w

R=adamk@chromium.org

Bug: v8:10787
Change-Id: I9060c5cdc9dbafeb7ea7c5c26d09c2dc744800bd
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/2342847Reviewed-by: Adam Klein <adamk@chromium.org>
Commit-Queue: Andreas Haas <ahaas@chromium.org>
Cr-Commit-Position: refs/heads/master@{#69312}