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Previously, CryptAuthEnrollmentManager::Observer was a pure-virtual
class; however, not all observers override all observer functions,
resulting in the need to define empty callback implementations for
all derived classes.

This change is in preparation for adding yet another observer which does
not need to override all observer functions.

Bug: 824568, 752273
Change-Id: I036c720f0331167a15fb0a36e1f4e7c19a6ac356
Reviewed-on: https://chromium-review.googlesource.com/994288
Commit-Queue: Kyle Horimoto <khorimoto@chromium.org>
Commit-Queue: Ryan Hansberry <hansberry@chromium.org>
Reviewed-by: Ryan Hansberry <hansberry@chromium.org>
Cr-Commit-Position: refs/heads/master@{#547920}