Policy definition for encryptfs to ext4 migration strategy

This is just the proto definition for the policy. It should allow
development on server side without waiting for client implementation.

BUG=722371

Review-Url: https://codereview.chromium.org/2885153005
Cr-Original-Commit-Position: refs/heads/master@{#472487}
Cr-Mirrored-From: https://chromium.googlesource.com/chromium/src
Cr-Mirrored-Commit: 3f2e3a76424261598c4033dcb06e49f9c3205808