Fix mirror-script and debug-script tests when using external natives.
Fix mirror-script and debug-script tests when using external natives.

R=yangguo
BUG=

Review URL: https://codereview.chromium.org/792733003

Cr-Commit-Position: 972c6d2dc6dd5efdad1377c0d224e03eb8f276f7@{#25752}

UniversalFuture/moosh,UniversalFuture/moosh,UniversalFuture/moosh,UniversalFuture/moosh