Remove the requirement to pass the native context into the script cache,
simple to be able to access the empty function. Instead, for script
cache keys, use Smi::zero() in the 'owner function' slot.

This allows CompileUnboundScript to be called outside of a Context
scope.

Change-Id: I9b4fe6dd43f14944728664f7203b748ced750e76
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/2794440
Commit-Queue: Leszek Swirski <leszeks@chromium.org>
Auto-Submit: Leszek Swirski <leszeks@chromium.org>
Reviewed-by: Camillo Bruni <cbruni@chromium.org>
Cr-Commit-Position: refs/heads/master@{#73737}