The plan is to also use it for With and Catch scopes, so all kinds of
contexts have a pointer back to their ScopeInfo

R=neis@chromium.org,marja@chromium.org
BUG=v8:5215

Review-Url: https://codereview.chromium.org/2301913002
Cr-Commit-Position: refs/heads/master@{#39092}