This removes a lot of special handling for the predictable platform.
Instead of executing spawned foreground and background tasks
immediately (i.e. inside the scope that spawns the tasks), just add
both to the foreground task queue.

This avoids existing special handling for predictable mode in wasm
async compilation, and should fix current failures on the predictable
bot.

BUG=v8:6427

Change-Id: Idbaa764a3dc8c230c29f3937d885e12174691ac4
Reviewed-on: https://chromium-review.googlesource.com/509694Reviewed-by: Jochen Eisinger <jochen@chromium.org>
Commit-Queue: Clemens Hammacher <clemensh@chromium.org>
Cr-Commit-Position: refs/heads/master@{#45538}