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The Account ID of the account for which the user granted consent
is passed from:
- Desktop: SyncConfirmationHandler
- Android: ConsentTracker
- iOS: ChromeSigninViewController
- ChromeOS: ArcSupportHost, ArcTermsOfServiceScreenHandler

to ConsentAuditor. In this CL, we only verify that it's not empty.

A subsequent CL will pass it to UserEventService which will verify that
it's the same account that is currently signed in, to avoid sending
consents for account A over an authenticated channel of account B
(and associating the consent with B on the server).

Bug: 781765
Cq-Include-Trybots: master.tryserver.chromium.mac:ios-simulator-cronet;master.tryserver.chromium.mac:ios-simulator-full-configs
Change-Id: I5179b8932a3ec8fd865e97c2f8164ac2715ac840
Reviewed-on: https://chromium-review.googlesource.com/986293
Commit-Queue: Martin Šrámek <msramek@chromium.org>
Reviewed-by: Bernhard Bauer <bauerb@chromium.org>
Reviewed-by: Xiyuan Xia <xiyuan@chromium.org>
Reviewed-by: Boris Sazonov <bsazonov@chromium.org>
Reviewed-by: Christian Dullweber <dullweber@chromium.org>
Reviewed-by: Mihai Sardarescu <msarda@chromium.org>
Cr-Commit-Position: refs/heads/master@{#549870}