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Chrome OS Account Manager has been launched to Stable channel and
enabled on ToT. Cleanup the flag in Chrome signin code.

Bug: 1009960
Test: unit_tests --gtest_filter="*ChromeSigninHelperTest*"
Change-Id: I012a6967f890febba66e1ce8e19764b46e357aee
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1833609
Commit-Queue: Kush Sinha <sinhak@chromium.org>
Reviewed-by: Boris Sazonov <bsazonov@chromium.org>
Cr-Commit-Position: refs/heads/master@{#702042}