Allow Safe Browsing backend to select downloads to upload.

This gives the server more nuanced control so it can mark a file as
verdict UNKNOWN but still request it get uploaded, to improve the
verdict for later.

BUG=687685

Review-Url: https://codereview.chromium.org/2696973002
Cr-Commit-Position: refs/heads/master@{#452181}
8 files changed