The predictable platform only executed background tasks if at least one
foreground task was executed. Async compilation in Wasm only spawns a
background task though, so that one could be missed.

This CL fixes the loop to also execute background tasks if no foreground
task was executed.

R=ahaas@chromium.org

Bug: v8:11848
Change-Id: Ia0b32427c24a79d5710c784b98528bf431471528
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/2944833Reviewed-by: Andreas Haas <ahaas@chromium.org>
Commit-Queue: Clemens Backes <clemensb@chromium.org>
Cr-Commit-Position: refs/heads/master@{#75007}