We need to ensure that the NativeModule stays alive while any
{BackgroundCompileScope} exists, because during that time we hold
shared ownership of the mutex in the {BackgroundCompileToken}. If the
{NativeModule} dies during that period, we would need to get exclusive
ownership of the mutex and deadlock.

This change requires holding a {std::weak_ptr<NativeModule>} in the
BackgroundCompileToken instead of a raw pointer, hence it can only be
initialized after the NativeModule was created. This is done via a
separate {InitCompilationState} method.

R=ahaas@chromium.org

Bug: v8:8979
Change-Id: Ia14bd272ea0bc47aec547024da6020608418c9d2
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/1518178
Auto-Submit: Clemens Hammacher <clemensh@chromium.org>
Reviewed-by: Andreas Haas <ahaas@chromium.org>
Commit-Queue: Clemens Hammacher <clemensh@chromium.org>
Cr-Commit-Position: refs/heads/master@{#60203}