Retire ScopedObserver in /components/suggestions.

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=mathp@chromium.org

Bug: 1145565
Change-Id: Ia01c31b800dbb549e6b7daf7636b055c7f559111
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2532958
Auto-Submit: Sigurður Ásgeirsson <siggi@chromium.org>
Reviewed-by: Marc Treib <treib@chromium.org>
Commit-Queue: Sigurður Ásgeirsson <siggi@chromium.org>
Cr-Commit-Position: refs/heads/master@{#827810}

components/suggestions/suggestions_service_impl.cc

@@ -121,7 +121,7 @@ SuggestionsServiceImpl::SuggestionsServiceImpl(
       blocklist_upload_timer_(tick_clock_) {
   // |sync_service_| is null if switches::kDisableSync is set (tests use that).
   if (sync_service_)
-    sync_service_observer_.Add(sync_service_);
+    sync_service_observation_.Observe(sync_service_);
   // Immediately get the current sync state, so we'll flush the cache if
   // necessary.
   OnStateChanged(sync_service_);
@@ -522,7 +522,8 @@ void SuggestionsServiceImpl::Shutdown() {
   // Cancel pending request.
   pending_request_.reset(nullptr);
 
-  sync_service_observer_.RemoveAll();
+  if (sync_service_observation_.IsObserving())
+    sync_service_observation_.RemoveObservation();
 }
 
 void SuggestionsServiceImpl::ScheduleBlocklistUpload() {

components/suggestions/suggestions_service_impl.h

@@ -16,7 +16,7 @@
 #include "base/macros.h"
 #include "base/memory/weak_ptr.h"
 #include "base/optional.h"
-#include "base/scoped_observer.h"
+#include "base/scoped_observation.h"
 #include "base/threading/thread_checker.h"
 #include "base/time/tick_clock.h"
 #include "base/time/time.h"
@@ -174,8 +174,8 @@ class SuggestionsServiceImpl : public SuggestionsService,
   signin::IdentityManager* identity_manager_;
 
   syncer::SyncService* sync_service_;
-  ScopedObserver<syncer::SyncService, syncer::SyncServiceObserver>
-      sync_service_observer_{this};
+  base::ScopedObservation<syncer::SyncService, syncer::SyncServiceObserver>
+      sync_service_observation_{this};
 
   // The state of history sync, i.e. are we uploading history data to Google?
   syncer::UploadState history_sync_state_;