Adding a SQL implementation of the backing store for previews opt outs

components/previews/core/previews_black_list.h

Index: components/previews/core/previews_black_list.h
diff --git a/components/previews/core/previews_black_list.h b/components/previews/core/previews_black_list.h
index 40b50d58788e8bb8ca5e0eeb9e8e3c5ff46aa69e..92cc3bccf4995b563d5df2258fc8290dbb638581 100644
--- a/components/previews/core/previews_black_list.h
+++ b/components/previews/core/previews_black_list.h
@@ -39,20 +39,23 @@ class PreviewsBlackListItem;
 class PreviewsBlackList {
  public:
   // |opt_out_store| is the backing store to retrieve and store black list
   // information, and can be null. When |opt_out_store| is null, the in-memory
   // map will be immediately loaded to empty. If |opt_out_store| is non-null,
   // it will be used to load the in-memory map asynchronously.
   PreviewsBlackList(std::unique_ptr<PreviewsOptOutStore> opt_out_store,
                     std::unique_ptr<base::Clock> clock);
   ~PreviewsBlackList();
 
+  // Begins loading the black list from the backing store if one was passed in.
+  void Initialize();
+
   // Asynchronously adds a new navigation to to the in-memory black list and
   // backing store. |opt_out| is whether the user opted out of the preview or
   // navigated away from the page without opting out. |type| is only passed to
   // the backing store. If the in memory map has reached the max number of hosts
   // allowed, and |url| is a new host, a host will be evicted based on recency
   // of the hosts most recent opt out.
   void AddPreviewNavigation(const GURL& url, bool opt_out, PreviewsType type);
 
   // Synchronously determines if |host_name| should be allowed to show previews.
   // If the black list has loaded yet, this will always return false. |type| is