Retire ScopedObserver in /components/services/app_service.

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=nancylingwang@chromium.org

Bug: 1145565
Change-Id: Ia8d2596dcd98bf0762357de363585affa59dff57
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2532722
Auto-Submit: Sigurður Ásgeirsson <siggi@chromium.org>
Reviewed-by: Nancy Wang <nancylingwang@chromium.org>
Commit-Queue: Nancy Wang <nancylingwang@chromium.org>
Cr-Commit-Position: refs/heads/master@{#826480}

components/services/app_service/public/cpp/app_registry_cache.h

@@ -43,7 +43,7 @@ class COMPONENT_EXPORT(APP_UPDATE) AppRegistryCache {
     // Called when the AppRegistryCache object (the thing that this observer
     // observes) will be destroyed. In response, the observer, |this|, should
     // call "cache->RemoveObserver(this)", whether directly or indirectly (e.g.
-    // via ScopedObserver::Remove or via Observe(nullptr)).
+    // via base::ScopedObservation::Remove or via Observe(nullptr)).
     virtual void OnAppRegistryCacheWillBeDestroyed(AppRegistryCache* cache) = 0;
 
    protected:

components/services/app_service/public/cpp/instance_registry.h

@@ -40,7 +40,8 @@ class InstanceRegistry {
     // Called when the InstanceRegistry object (the thing that this observer
     // observes) will be destroyed. In response, the observer, |this|, should
     // call "instance_registry->RemoveObserver(this)", whether directly or
-    // indirectly (e.g. via ScopedObserver::Remove or via Observe(nullptr)).
+    // indirectly (e.g. via base::ScopedObservation::Remove or via
+    // Observe(nullptr)).
     virtual void OnInstanceRegistryWillBeDestroyed(InstanceRegistry* cache) = 0;
 
    protected: