This fixes a gc stress bug. We cannot rely on an ordering of
clearing of the weak cells, so we explicitly reset the weak
link to the owning instance, when finalizing a compiled
module. In turn, this serves as a reliable signal when GCs
happen while instantiating, allowing us to correctly link the
new instance.

BUG=chromium:652425

Review-Url: https://codereview.chromium.org/2393443003
Cr-Commit-Position: refs/heads/master@{#39964}