Safely cancel prerenders on threads other than the UI thread

chrome/chrome_browser.gypi

 # Copyright (c) 2011 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.
 
 {
   'targets': [
     {
       'target_name': 'browser',
       'type': '<(library)',
       'msvs_guid': '5BF908A7-68FB-4A4B-99E3-8C749F1FE4EA',
@@ skipping 1623 matching lines @@
         'browser/prerender/prerender_final_status.cc',
         'browser/prerender/prerender_final_status.h',
         'browser/prerender/prerender_manager.cc',
         'browser/prerender/prerender_manager.h',
         'browser/prerender/prerender_observer.cc',
         'browser/prerender/prerender_observer.h',
         'browser/prerender/prerender_render_view_host_observer.cc',
         'browser/prerender/prerender_render_view_host_observer.h',
         'browser/prerender/prerender_render_widget_host_view.cc',
         'browser/prerender/prerender_render_widget_host_view.h',
+        'browser/prerender/prerender_tracker.cc',
+        'browser/prerender/prerender_tracker.h',
         'browser/printing/background_printing_manager.cc',
         'browser/printing/background_printing_manager.h',
         'browser/printing/cloud_print/cloud_print_proxy_service.cc',
         'browser/printing/cloud_print/cloud_print_proxy_service.h',
         'browser/printing/cloud_print/cloud_print_setup_flow.cc',
         'browser/printing/cloud_print/cloud_print_setup_flow.h',
         'browser/printing/cloud_print/cloud_print_setup_handler.cc',
         'browser/printing/cloud_print/cloud_print_setup_handler.h',
         'browser/printing/cloud_print/cloud_print_setup_message_handler.cc',
         'browser/printing/cloud_print/cloud_print_setup_message_handler.h',
@@ skipping 2920 matching lines @@
       'hard_dependency': 1,
     },
   ],
 }
 
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 # tab-width:2
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