Jobs were still being posted on the underlying default platform, which
caused concurrent execution. By directly returning a
{NewDefaultJobHandle} with a pointer to the {PredictablePlatform}, we
force execution of all posted tasks via that platform.

R=ahaas@chromium.org, cbruni@chromium.org

Bug: v8:11848
Change-Id: Ie10519583341b427776ca428f85641e96f821367
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/2944808Reviewed-by: Camillo Bruni <cbruni@chromium.org>
Reviewed-by: Andreas Haas <ahaas@chromium.org>
Commit-Queue: Clemens Backes <clemensb@chromium.org>
Cr-Commit-Position: refs/heads/master@{#75002}