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When announcing the availability of the robot account token to its
observers, DeviceOAuth2TokenService needs to know both the token and
the robot account ID. During enrollment, the token becomes available
before the account ID. Thus, DeviceOAuth2TokenService needs to wait
for the account ID to be available before announcing the token.

BUG=453828
TEST=New unit test

Review URL: https://codereview.chromium.org/892633003

Cr-Commit-Position: refs/heads/master@{#314312}