There might be several ExternalCallbackScope's created
during the native callback. Remove the assert that is not
aligned with that.

Moreover this iterator must work for any kind of
stacks including corrupted ones.

BUG=v8:4705
LOG=N

Review URL: https://codereview.chromium.org/1663193003

Cr-Commit-Position: refs/heads/master@{#33751}