Retire ScopedObserver in /components/undo.

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=tom.cassiotis@gmail.com

Bug: 1145565
Change-Id: I8c5915322326c6ed819025f922af9432aabc08be
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2532660
Auto-Submit: Sigurður Ásgeirsson <siggi@chromium.org>
Commit-Queue: Cait Phillips <caitkp@chromium.org>
Reviewed-by: Cait Phillips <caitkp@chromium.org>
Cr-Commit-Position: refs/heads/master@{#828306}

components/undo/bookmark_undo_service.cc

@@ -336,13 +336,13 @@ BookmarkUndoService::~BookmarkUndoService() {
 void BookmarkUndoService::Start(BookmarkModel* model) {
   DCHECK(!model_);
   model_ = model;
-  scoped_observer_.Add(model);
+  scoped_observation_.Observe(model);
   model->SetUndoDelegate(this);
 }
 
 void BookmarkUndoService::Shutdown() {
   DCHECK(model_);
-  scoped_observer_.RemoveAll();
+  scoped_observation_.RemoveObservation();
   model_->SetUndoDelegate(nullptr);
 }
 

components/undo/bookmark_undo_service.h

@@ -8,7 +8,7 @@
 #include <map>
 
 #include "base/macros.h"
-#include "base/scoped_observer.h"
+#include "base/scoped_observation.h"
 #include "components/bookmarks/browser/base_bookmark_model_observer.h"
 #include "components/bookmarks/browser/bookmark_model.h"
 #include "components/bookmarks/browser/bookmark_node_data.h"
@@ -70,8 +70,9 @@ class BookmarkUndoService : public bookmarks::BaseBookmarkModelObserver,
   bookmarks::BookmarkModel* model_;
   bookmarks::BookmarkUndoProvider* undo_provider_;
   UndoManager undo_manager_;
-  ScopedObserver<bookmarks::BookmarkModel, bookmarks::BookmarkModelObserver>
-      scoped_observer_{this};
+  base::ScopedObservation<bookmarks::BookmarkModel,
+                          bookmarks::BookmarkModelObserver>
+      scoped_observation_{this};
 
   DISALLOW_COPY_AND_ASSIGN(BookmarkUndoService);
 };