Keep track of script objects in a weak fixed array.

Keep track of script objects in a weak fixed array.

We need this for the debugger and for future changes that need to
find all shared function infos (through scripts).

R=mvstanton@chromium.org

Review URL: https://codereview.chromium.org/1297273005

Cr-Commit-Position: refs/heads/master@{#30264}