Adding ClearBlackList to the PreviewsBlackList and plumbing to UI

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..13099e5b8cd77a68de4dcd74beceff630d06b268 100644
--- a/components/previews/core/previews_black_list.h
+++ b/components/previews/core/previews_black_list.h
@@ -52,30 +52,39 @@ class PreviewsBlackList {
   // 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
   // not used to make this decision.
   bool IsLoadedAndAllowed(const GURL& url, PreviewsType type) const;
 
+  // Asynchronously deletes all entries in the in-memory black list. Informs
+  // the backing store to delete entries between |begin_time| and |end_time|.
+  // Reloads entries into memory from the backing store. If the embedder passed
+  // in a null store, resets all history in the in-memory black list.

[tbansal1, 2016/10/03 16:48:12]

Slightly unclear. May be:
s/|end_time|. Reloads/ |end_time|, and reloads/

[RyanSturm, 2016/10/03 17:23:13]

Done.

+  void ClearBlackList(base::Time begin_time, base::Time end_time);
+
  private:
   // Synchronous version of AddPreviewNavigation method.
   void AddPreviewNavigationSync(const GURL& host_name,
                                 bool opt_out,
                                 PreviewsType type);
 
   // Returns the PreviewsBlackListItem representing |host_name|. If there is no
   // item for |host_name|, returns null.
   PreviewsBlackListItem* GetBlackListItem(const std::string& host_name) const;
 
+  // Synchronous version of ClearBlackList method.
+  void ClearBlackListSync(base::Time begin_time, base::Time end_time);
+
   // Returns a new PreviewsBlackListItem representing |host_name|. Adds the new
   // item to the in-memory map.
   PreviewsBlackListItem* CreateBlackListItem(const std::string& host_name);
 
   // Callback passed to the backing store when loading black list information.
   // Moves the returned map into the in-memory black list and runs any
   // outstanding tasks.
   void LoadBlackListDone(std::unique_ptr<BlackListItemMap> black_list_item_map);
 
   // Called while waiting for the black list to be loaded from the backing