Retire ScopedObserver in /components/sync.

ScopedObserver is being deprecated in favor of two new classes:
- base::ScopedObservation for observers that only ever observe
  a single source.
- base::ScopedMultiSourceObservation for observers that do or may
  observe more than a single source.

This CL was uploaded by git cl split.

R=rushans@google.com

Bug: 1145565
Change-Id: I760ad7d03410e304b00db5497fb44120e9768fd0
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2532959
Auto-Submit: Sigurður Ásgeirsson <siggi@chromium.org>
Commit-Queue: Rushan Suleymanov <rushans@google.com>
Reviewed-by: Rushan Suleymanov <rushans@google.com>
Cr-Commit-Position: refs/heads/master@{#827855}

components/sync/driver/sync_session_durations_metrics_recorder.cc

@@ -30,9 +30,9 @@ SyncSessionDurationsMetricsRecorder::SyncSessionDurationsMetricsRecorder(
     : sync_service_(sync_service), identity_manager_(identity_manager) {
   // |sync_service| can be null if sync is disabled by a command line flag.
   if (sync_service_) {
-    sync_observer_.Add(sync_service_);
+    sync_observation_.Observe(sync_service_);
   }
-  identity_manager_observer_.Add(identity_manager_);
+  identity_manager_observation_.Observe(identity_manager_);
 
   // Since this is created after the profile itself is created, we need to
   // handle the initial state.
@@ -52,8 +52,9 @@ SyncSessionDurationsMetricsRecorder::SyncSessionDurationsMetricsRecorder(
 
 SyncSessionDurationsMetricsRecorder::~SyncSessionDurationsMetricsRecorder() {
   DCHECK(!total_session_timer_) << "Missing a call to OnSessionEnded().";
-  sync_observer_.RemoveAll();
-  identity_manager_observer_.RemoveAll();
+  if (sync_observation_.IsObserving())
+    sync_observation_.RemoveObservation();
+  identity_manager_observation_.RemoveObservation();
 }
 
 void SyncSessionDurationsMetricsRecorder::OnSessionStarted(

components/sync/driver/sync_session_durations_metrics_recorder.h

@@ -9,7 +9,7 @@
 #include <string>
 #include <vector>
 
-#include "base/scoped_observer.h"
+#include "base/scoped_observation.h"
 #include "base/timer/elapsed_timer.h"
 #include "components/keyed_service/core/keyed_service.h"
 #include "components/signin/public/identity_manager/identity_manager.h"
@@ -74,10 +74,11 @@ class SyncSessionDurationsMetricsRecorder
   SyncService* const sync_service_;
   signin::IdentityManager* const identity_manager_;
 
-  ScopedObserver<syncer::SyncService, syncer::SyncServiceObserver>
-      sync_observer_{this};
-  ScopedObserver<signin::IdentityManager, signin::IdentityManager::Observer>
-      identity_manager_observer_{this};
+  base::ScopedObservation<syncer::SyncService, syncer::SyncServiceObserver>
+      sync_observation_{this};
+  base::ScopedObservation<signin::IdentityManager,
+                          signin::IdentityManager::Observer>
+      identity_manager_observation_{this};
 
   // Tracks the elapsed active session time while the browser is open. The timer
   // is absent if there's no active session.