Replace refcounting with weak pointers for BookmarkStore.

components/bookmarks/browser/bookmark_model.cc

Index: components/bookmarks/browser/bookmark_model.cc
diff --git a/components/bookmarks/browser/bookmark_model.cc b/components/bookmarks/browser/bookmark_model.cc
index 13cd1ee1ae1dc163e39eb69a4f09273a92839ede..df347fb339977469d922ae79c7674fb24d891c68 100644
--- a/components/bookmarks/browser/bookmark_model.cc
+++ b/components/bookmarks/browser/bookmark_model.cc
@@ -147,7 +147,7 @@ void BookmarkModel::Load(
       new BookmarkExpandedStateTracker(this, pref_service));
 
   // Load the bookmarks. BookmarkStorage notifies us when done.
-  store_ = new BookmarkStorage(this, profile_path, io_task_runner.get());
+  store_ .reset(new BookmarkStorage(this, profile_path, io_task_runner.get()));
   store_->LoadBookmarks(CreateLoadDetails(accept_languages), ui_task_runner);
 }
 
@@ -700,7 +700,7 @@ void BookmarkModel::GetBookmarksMatching(
 }
 
 void BookmarkModel::ClearStore() {
-  store_ = NULL;
+  store_.reset();
 }
 
 void BookmarkModel::SetPermanentNodeVisible(BookmarkNode::Type type,