Add option to disallow the discarding of a tab that was previously discarded.

chrome/chrome_browser_ui.gypi

 # Copyright 2012 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.
 
 {
   'variables': {
     # There are two sections of sources:
     # * "Platform"-based sources (OS, toolkit, etc.)
     # * Feature/directory-based sources (e.g., history, printing)
 
@@ skipping 1668 matching lines @@
       'browser/ui/tab_modal_confirm_dialog_delegate.cc',
       'browser/ui/tab_modal_confirm_dialog_delegate.h',
       'browser/ui/tabs/hover_tab_selector.cc',
       'browser/ui/tabs/hover_tab_selector.h',
       'browser/ui/tabs/pinned_tab_codec.cc',
       'browser/ui/tabs/pinned_tab_codec.h',
       'browser/ui/tabs/pinned_tab_service.cc',
       'browser/ui/tabs/pinned_tab_service.h',
       'browser/ui/tabs/pinned_tab_service_factory.cc',
       'browser/ui/tabs/pinned_tab_service_factory.h',
+      'browser/ui/tabs/tab_discard_state.cc',
+      'browser/ui/tabs/tab_discard_state.h',
       'browser/ui/tabs/tab_menu_model.cc',
       'browser/ui/tabs/tab_menu_model.h',
       'browser/ui/tabs/tab_resources.cc',
       'browser/ui/tabs/tab_resources.h',
       'browser/ui/tabs/tab_strip_model.cc',
       'browser/ui/tabs/tab_strip_model.h',
       'browser/ui/tabs/tab_strip_model_delegate.h',
       'browser/ui/tabs/tab_strip_model_observer.cc',
       'browser/ui/tabs/tab_strip_model_observer.h',
       'browser/ui/tabs/tab_strip_model_order_controller.cc',
@@ skipping 1504 matching lines @@
           ],
           'sources': [
             '<@(chrome_browser_ui_non_mobile_sources)',
             '<@(chrome_browser_ui_omnibox_non_mobile_sources)',
           ],
         }],
       ],
     },
   ],
 }