Add code to compute the root hash for a hash tree

extensions/extensions.gyp

 # Copyright 2013 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': {
     'chromium_code': 1,
   },
   'targets': [
     {
@@ skipping 298 matching lines @@
         'browser/api_activity_monitor.h',
         'browser/app_sorting.h',
         'browser/blacklist_state.h',
         'browser/browser_context_keyed_api_factory.h',
         'browser/browser_context_keyed_service_factories.cc',
         'browser/browser_context_keyed_service_factories.h',
         'browser/content_hash_fetcher.cc',
         'browser/content_hash_fetcher.h',
         'browser/content_hash_reader.cc',
         'browser/content_hash_reader.h',
+        'browser/content_hash_tree.cc',
+        'browser/content_hash_tree.h',
         'browser/content_verifier.cc',
         'browser/content_verifier.h',
         'browser/content_verifier_delegate.h',
         'browser/content_verify_job.cc',
         'browser/content_verify_job.h',
         'browser/error_map.cc',
         'browser/error_map.h',
         'browser/event_listener_map.cc',
         'browser/event_listener_map.h',
         'browser/event_router.cc',
@@ skipping 298 matching lines @@
         'test/test_extensions_client.cc',
         'test/test_extensions_client.h',
         'test/test_permission_message_provider.cc',
         'test/test_permission_message_provider.h',
         'test/test_permissions_provider.cc',
         'test/test_permissions_provider.h',
       ],
     },
   ]
 }