[interpreter] DYNAMIC_GLOBAL needs to walk the context chain starting from current_scope, not closure_scope

The main reason why we currently don't see this fail is that block-scopes always appear to have an extension: the scope info object is stored there.

Bug: 
Change-Id: I38f0c15387e235eeea9a57c95af0d9eb185dad2a
Reviewed-on: https://chromium-review.googlesource.com/785951
Commit-Queue: Toon Verwaest <verwaest@chromium.org>
Reviewed-by: Ross McIlroy <rmcilroy@chromium.org>
Cr-Commit-Position: refs/heads/master@{#49585}