Reland: Add IdleHint function

The initial version of this CL was reverted because the CLI version
wasn't working. This now seems to be working locally in the browser
and through CLI.

The difference now is that we set idleHint also on the Realm we create
for running benchmarks in (globalThis.top.idleHint).

Change-Id: I7f66cf9d376d491085a11aa304cad9bffcc17904
Reviewed-on: https://chromium-review.googlesource.com/c/external/github.com/WebKit/JetStream/+/7481050
Reviewed-by: Camillo Bruni <cbruni@chromium.org>
Reviewed-by: Patrick Thier <pthier@chromium.org>