Last_n: Add instrumentation test for the tab closure case.

chrome/browser/android/offline_pages/recent_tab_helper.cc

 // Copyright (c) 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/android/offline_pages/recent_tab_helper.h"
 
 #include <queue>
 #include <vector>
 
 #include "base/bind.h"
@@ skipping 243 matching lines @@
   if (downloads_ongoing_snapshot_info_) {
     DVLOG(1) << "WebContents destroyed; passing ongoing downloads request to "
                 "Background Offliner";
     ReportDownloadStatusToRequestCoordinator(
         downloads_ongoing_snapshot_info_.get(), false);
   }
   // And cancel any ongoing snapshots.
   CancelInFlightSnapshots();
 }
 
-// TODO(carlosk): this method is also called when the tab is being closed, when
-// saving a snapshot is probably useless (low probability of the user undoing
-// the close). We should detect that and avoid the saving.
 void RecentTabHelper::WasHidden() {
   if (!IsOffliningRecentPagesEnabled())
     return;
 
   // Return immediately if last_n is not listening to tab hidden events, if a
   // last_n snapshot is currently being saved or if the tab is closing.
   if (!last_n_listen_to_tab_hidden_ || last_n_ongoing_snapshot_info_ ||
       tab_is_closing_) {
     DVLOG(1) << "Will not snapshot for last_n (reasons: "
              << !last_n_listen_to_tab_hidden_ << ", "
@@ skipping 202 matching lines @@
 }
 
 void RecentTabHelper::CancelInFlightSnapshots() {
   weak_ptr_factory_.InvalidateWeakPtrs();
   downloads_ongoing_snapshot_info_.reset();
   downloads_latest_saved_snapshot_info_.reset();
   last_n_ongoing_snapshot_info_.reset();
 }
 
 }  // namespace offline_pages