Since the extension field is already used for the catch name, store a
ContextExtension there instead.

In the future, this will allow for chaining ScopeInfos together, so we
no longer need a context chain for lazy parsing / compilation.

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

Review-Url: https://codereview.chromium.org/2302013002
Cr-Commit-Position: refs/heads/master@{#39164}