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It seems that the server currently has logic to not send back excluded
IDs, but lacks the logic needed to request more. Still, that's something
that can be improved in the server later.

There should be tests for NTPSnippetsService to verify the new behavior,
but that's not possible until it uses the new server.

BUG=633613

Review-Url: https://codereview.chromium.org/2274953002
Cr-Commit-Position: refs/heads/master@{#414428}