Instead of modifying a context chain and then modifying it back, causing
potential mismatches, we clone the inner context chain and evaluate
the expression in this cloned context. We then copy all local variable
values back if needed.

R=yangguo@chromium.org,yurys@chromium.org

Review URL: https://codereview.chromium.org/1088503003

Cr-Commit-Position: refs/heads/master@{#27809}