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Implement a new OAuth2 delegate for Chrome OS which uses Chrome OS
Account Manager as the source of truth for OAuth credentials.

Bug: 820046
Test: unit_tests --gtest_filter="*CrOSOAuthDelegateTest*"
Change-Id: I5b9dde8bc988f3aa1edf27b06691b07d2a7d5939
Reviewed-on: https://chromium-review.googlesource.com/1014041Reviewed-by: Mihai Sardarescu <msarda@chromium.org>
Reviewed-by: Xiyuan Xia <xiyuan@chromium.org>
Reviewed-by: Lutz Justen <ljusten@chromium.org>
Reviewed-by: David Roger <droger@chromium.org>
Commit-Queue: Kush Sinha <sinhak@chromium.org>
Cr-Commit-Position: refs/heads/master@{#555335}