The motivation here is to support sampling clients. When a client isn't in the sample, reporting will be disabled, even if they've consented.

Knowing consent is still important to make sure the client_id is created, and the high entropy source is used for variations selection. Otherwise, clients would be shuffled into different variations whenever they come in or out of the sample.

BUG=609987

Review-Url: https://codereview.chromium.org/1958003003
Cr-Commit-Position: refs/heads/master@{#393333}