Add DCHECK that the script context table do not contain native scripts.

Add DCHECK that the script context table do not contain native scripts.

This is to prevent unnecessary script contexts to be created for
native scripts, which could end up with a script context table
containing twenty script contexts for a newly created native context.

R=jkummerow@chromium.org

Review URL: https://codereview.chromium.org/1301533002

Cr-Commit-Position: refs/heads/master@{#30188}