Turns out is_hidden is not the right condition for "scope should be present in
the preparse data". For now, replaced it with "is hidden leaf scope" (i.e.,
doesn't contain any non-hidden scopes). That's probably not the right condition
either; will be fixed once there's more data to decide what the right condition
is.

BUG=v8:5516
R=vogelheim@chromium.org

Review-Url: https://codereview.chromium.org/2669163002
Cr-Commit-Position: refs/heads/master@{#42909}