If we use ScopeIterator inside a debug-evaluate call, we may iterate
over a debug-evaluate context that we created for the debug-evaluate
call. This may trigger assertions.

The solution is to have the ScopeIterator hide debug-evaluate contexts
by unwrapping it if it comes across any.

R=cbruni@chromium.org
BUG=chromium:599662
LOG=N

Review URL: https://codereview.chromium.org/1859033002

Cr-Commit-Position: refs/heads/master@{#35258}