Replace the zonelist with a link from a scope to any of its inner scopes, and a link to any sibling scope.
This makes scopes that track inner scopes use roughly the same amount of space as previously scopes without inner scopes would use for the empty zonelist (pointer to the memory + length field, which, granted could be slightly smaller on 64bit).

BUG=v8:5209

Review-Url: https://codereview.chromium.org/2162143005
Cr-Commit-Position: refs/heads/master@{#37941}