Top level SharedFunctionInfos will end up in a scripts SFI list, but
eval'd SFIs shouldn't. Separate IDs will allow for adding a
corresponding DCHECK.

BUG=v8:5589
R=marja@chromium.org

Review-Url: https://codereview.chromium.org/2533303006
Cr-Commit-Position: refs/heads/master@{#41421}