[CrOS MultiDevice] Adjust CryptAuthEnrollmentManager::Observer.

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}

chrome/browser/chromeos/cryptauth/chrome_cryptauth_service.cc

@@ -198,8 +198,6 @@ ChromeCryptAuthService::CreateCryptAuthClientFactory() {
   return CreateCryptAuthClientFactoryImpl(profile_);
 }
 
-void ChromeCryptAuthService::OnEnrollmentStarted() {}
-
 void ChromeCryptAuthService::OnEnrollmentFinished(bool success) {
   if (success)
     device_manager_->Start();

chrome/browser/chromeos/cryptauth/chrome_cryptauth_service.h

@@ -53,7 +53,6 @@ class ChromeCryptAuthService
   CreateSecureMessageDelegate() override;
 
   // cryptauth::CryptAuthEnrollmentManager::Observer:
-  void OnEnrollmentStarted() override;
   void OnEnrollmentFinished(bool success) override;
 
  protected:

components/cryptauth/cryptauth_enrollment_manager.h

@@ -25,11 +25,11 @@ class CryptAuthEnrollmentManager {
   class Observer {
    public:
     // Called when an enrollment attempt is started.
-    virtual void OnEnrollmentStarted() = 0;
+    virtual void OnEnrollmentStarted() {}
 
     // Called when an enrollment attempt finishes with the |success| of the
     // attempt.
-    virtual void OnEnrollmentFinished(bool success) = 0;
+    virtual void OnEnrollmentFinished(bool success) {}
 
     virtual ~Observer() = default;
   };