Make d8 default to standard location for external snapshots.
Make d8 default to standard location for external snapshots.

This makes tests runnable with the external snapshot, and should be the
last step before enabling external snapshot on >=1 bots.

R=yangguo
BUG=

Review URL: https://codereview.chromium.org/780333004

Cr-Commit-Position: 972c6d2dc6dd5efdad1377c0d224e03eb8f276f7@{#25753}

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