Now that ordered_variables_ is used to find non-dynamic variables, and NonLocals are always stored in the scope that introduces them, we can rely on variables_ to also cache non-locals. This has 2 advantages:
1) we don't need DynamicScopePart anymore, reducing all scopes by a pointer
2) upon second lookup of a non-local we don't need to walk the entire chain anymore. The cached value will immediately be found.

BUG=v8:5209

Review-Url: https://codereview.chromium.org/2276483003
Cr-Commit-Position: refs/heads/master@{#38855}