Fixes an invalidated pointer access in NTPSnipppetsFetcher.

chrome/browser/ntp_snippets/download_suggestions_provider.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 "chrome/browser/ntp_snippets/download_suggestions_provider.h"
 
 #include <algorithm>
 #include <utility>
 
 #include "base/bind.h"
@@ skipping 149 matching lines @@
   // available there.
   // TODO(vitaliii): Provide site's favicon for assets downloads. See
   // crbug.com/631447.
   base::ThreadTaskRunnerHandle::Get()->PostTask(
       FROM_HERE, base::Bind(callback, gfx::Image()));
 }
 
 void DownloadSuggestionsProvider::Fetch(
     const ntp_snippets::Category& category,
     const std::set<std::string>& known_suggestion_ids,
-    FetchingCallback callback) {
+    const FetchingCallback& callback) {
   NOTREACHED();
   base::ThreadTaskRunnerHandle::Get()->PostTask(
       FROM_HERE,
       base::Bind(callback, base::Passed(std::vector<ContentSuggestion>())));
 }
 
 void DownloadSuggestionsProvider::ClearHistory(
     base::Time begin,
     base::Time end,
     const base::Callback<bool(const GURL& url)>& filter) {
@@ skipping 457 matching lines @@
 
 void DownloadSuggestionsProvider::UnregisterDownloadItemObservers() {
   DCHECK_NE(download_manager_, nullptr);
 
   std::vector<DownloadItem*> all_downloads;
   download_manager_->GetAllDownloads(&all_downloads);
 
   for (DownloadItem* item : all_downloads)
     item->RemoveObserver(this);
 }