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This CL implements AffiliationBackend::GetAffiliation(), including support for caching, but without support for some error scenarios.

The missing parts, viz. Prefetch() functionality and error handling will be added later to reduce CL size. Given that the code is not yet exercised outside of tests, this should not create any problems.

BUG=437865
TBR=gcasto@chromium.org

Review URL: https://codereview.chromium.org/868253005

Cr-Commit-Position: refs/heads/master@{#314320}