Add ConsentAuditor::RecordGaiaConsent()

Add an empty method for consent recording to define the api.

Bug: 781765
Change-Id: I36663814a004ad07015218797683f78519606e37
Reviewed-on: https://chromium-review.googlesource.com/781500


Reviewed-by: Martin Šrámek <msramek@chromium.org>
Commit-Queue: Christian Dullweber <dullweber@chromium.org>
Cr-Commit-Position: refs/heads/master@{#518281}