Handle null callbacks in OnFaviconDownloaded

components/ntp_tiles/icon_cacher.h

 // Copyright 2017 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 #ifndef COMPONENTS_NTP_TILES_ICON_CACHER_H_
 #define COMPONENTS_NTP_TILES_ICON_CACHER_H_
 
 #include "base/callback.h"
 #include "components/ntp_tiles/popular_sites.h"
 
 namespace ntp_tiles {
 
 // Ensures that a Popular Sites icon is cached, downloading and saving it if
 // not.
 //
 // Does not provide any way to get a fetched favicon; use the FaviconService for
 // that. All this interface guarantees is that FaviconService will be able to
 // get you an icon (if it exists).
 class IconCacher {
  public:
   virtual ~IconCacher() = default;
 
-  // Fetches the icon if necessary. It invokes |icon_available| if a new icon
-  // was fetched. This is the only required callback.
+  // Fetches the icon if necessary. If a new icon was fetched, the optional
+  // |icon_available| callback will be invoked.
   // If there are preliminary icons (e.g. provided by static resources), the
-  // |preliminary_icon_available| callback will be invoked additionally.
+  // optional |preliminary_icon_available| callback will be invoked in addition.
   // TODO(fhorschig): In case we keep these, make them OnceClosures.
   virtual void StartFetch(PopularSites::Site site,
                           const base::Closure& icon_available,
                           const base::Closure& preliminary_icon_available) = 0;
 };
 
 }  // namespace ntp_tiles
 
 #endif  // COMPONENTS_NTP_TILES_ICON_CACHER_H_