This prepares for re-landing crrev.com/956373002

This pulls all decision about the snapshot [no|internal|external] into one rule. Previously, this logic was in separate places and not /quite/ the same, which causes build problems.

BUG=

Review URL: https://codereview.chromium.org/1016603004

Cr-Commit-Position: refs/heads/master@{#27523}