Fixes an invalidated pointer access in NTPSnipppetsFetcher.

components/ntp_snippets/physical_web_pages/physical_web_page_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 "components/ntp_snippets/physical_web_pages/physical_web_page_suggestions_provider.h"
 
 #include <utility>
 
 #include "base/bind.h"
 #include "base/strings/string_number_conversions.h"
@@ skipping 80 matching lines @@
     const ContentSuggestion::ID& suggestion_id,
     const ImageFetchedCallback& callback) {
   // TODO(vitaliii): Implement.
   base::ThreadTaskRunnerHandle::Get()->PostTask(
       FROM_HERE, base::Bind(callback, gfx::Image()));
 }
 
 void PhysicalWebPageSuggestionsProvider::Fetch(
     const 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 PhysicalWebPageSuggestionsProvider::ClearHistory(
     base::Time begin,
     base::Time end,
     const base::Callback<bool(const GURL& url)>& filter) {
@@ skipping 22 matching lines @@
 
 // Updates the |category_status_| and notifies the |observer_|, if necessary.
 void PhysicalWebPageSuggestionsProvider::NotifyStatusChanged(
     CategoryStatus new_status) {
   if (category_status_ == new_status) return;
   category_status_ = new_status;
   observer()->OnCategoryStatusChanged(this, provided_category_, new_status);
 }
 
 }  // namespace ntp_snippets