Use the DefaultSearchManager as the exclusive authority on DSE, ignoring Web Data.

chrome/browser/search_engines/default_search_manager.cc

 // Copyright 2014 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/search_engines/default_search_manager.h"
 
 #include <algorithm>
 #include <utility>
 
 #include "base/bind.h"
@@ skipping 110 matching lines @@
     return extension_default_search_.get();
   }
   if (prefs_default_search_) {
     if (source)
       *source = FROM_USER;
     return prefs_default_search_.get();
   }
 
   if (source)
     *source = FROM_FALLBACK;
-  return fallback_default_search_.get();
+  return TemplateURLService::fallback_search_engines_disabled() ?
+      NULL : fallback_default_search_.get();
 }
 
 DefaultSearchManager::Source
 DefaultSearchManager::GetDefaultSearchEngineSource() const {
   Source source;
   GetDefaultSearchEngine(&source);
   return source;
 }
 
 void DefaultSearchManager::SetUserSelectedDefaultSearchEngine(
@@ skipping 254 matching lines @@
   MergePrefsDataWithPrepopulated();
 }
 
 void DefaultSearchManager::NotifyObserver() {
   if (!change_observer_.is_null()) {
     Source source = FROM_FALLBACK;
     TemplateURLData* data = GetDefaultSearchEngine(&source);
     change_observer_.Run(data, source);
   }
 }