Change SigninClient::CreateGaiaAuthFetcher return type to unique_ptr

CreateGaiaAuthFetcher interface expects the caller to take ownership of the
returned pointer. This CL makes this fact obvious and self-documented by
changing return type to unique_ptr<GaiaAuthFetcher>.

BUG=702574

Review-Url: https://codereview.chromium.org/2757873002
Cr-Commit-Position: refs/heads/master@{#457793}
11 files changed