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By returning a signed simple challenge that can actually be parsed as SignedData, we allow callers of the FakeCryptohomeClient to extract the original data back and process it, making for better fake behavior and simpler tests.

Note that the signature is purposedly not verifiable in the FakeCryptohomeClient.

BUG=643245
TEST=chromeos_unittests; unit_tests and components_unittests also pass

Review-Url: https://codereview.chromium.org/2297193006
Cr-Commit-Position: refs/heads/master@{#417118}