Fix external-snapshot startup when snapshot is missing, but natives source is available
Fix external-snapshot startup when snapshot is missing, but natives source is available

R=vogelheim@chromium.org
BUG=

Review URL: https://codereview.chromium.org/1049383004

Cr-Commit-Position: 972c6d2dc6dd5efdad1377c0d224e03eb8f276f7@{#27573}

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