Handle null callbacks in OnFaviconDownloaded

components/ntp_tiles/icon_cacher_impl.cc

 // Copyright 2016 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.
 
 #include "components/ntp_tiles/icon_cacher_impl.h"
 
 #include <utility>
 
 #include "components/favicon/core/favicon_service.h"
 #include "components/favicon/core/favicon_util.h"
@@ skipping 31 matching lines @@
   // For images with multiple frames, prefer one of size 128x128px.
   image_fetcher_->SetDesiredImageFrameSize(gfx::Size(128, 128));
 }
 
 IconCacherImpl::~IconCacherImpl() = default;
 
 void IconCacherImpl::StartFetch(
     PopularSites::Site site,
     const base::Closure& icon_available,
     const base::Closure& preliminary_icon_available) {
-  DCHECK(!icon_available.is_null());
   favicon::GetFaviconImageForPageURL(
       favicon_service_, site.url, IconType(site),
       base::Bind(&IconCacherImpl::OnGetFaviconImageForPageURLFinished,
                  base::Unretained(this), std::move(site), icon_available,
                  preliminary_icon_available),
       &tracker_);
 }
 
 void IconCacherImpl::OnGetFaviconImageForPageURLFinished(
     PopularSites::Site site,
@@ skipping 15 matching lines @@
 
 void IconCacherImpl::OnFaviconDownloaded(PopularSites::Site site,
                                          const base::Closure& icon_available,
                                          const std::string& id,
                                          const gfx::Image& fetched_image) {
   if (fetched_image.IsEmpty()) {
     return;
   }
 
   SaveIconForSite(site, fetched_image);
-  icon_available.Run();
+  if (icon_available) {
+    icon_available.Run();
+  }
 }
 
 void IconCacherImpl::SaveIconForSite(const PopularSites::Site& site,
                                      gfx::Image image) {
   favicon_base::SetFaviconColorSpace(&image);
   favicon_service_->SetFavicons(site.url, IconURL(site), IconType(site), image);
 }
 
 bool IconCacherImpl::ProvideDefaultIcon(const PopularSites::Site& site) {
   if (site.default_icon_resource < 0) {
     return false;
   }
   SaveIconForSite(site, ResourceBundle::GetSharedInstance().GetNativeImageNamed(
                             site.default_icon_resource));
   return true;
 }
 
 }  // namespace ntp_tiles