This allows us to stop using a Symbol, set as the name of the Module's
SharedFunctionInfo, as our storage for a hash.

As part of this, centralize the code for generating a random, non-zero
hash code in one place (there were previously two copies of this code,
and I needed to call it from a third file).

BUG=v8:5483
TBR=jochen@chromium.org

Review-Url: https://codereview.chromium.org/2395233003
Cr-Commit-Position: refs/heads/master@{#40102}