Add a "guard" content settings type for chooser permissions.

components/content_settings/core/browser/content_settings_registry.cc

 // Copyright 2015 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 "components/content_settings/core/browser/content_settings_registry.h"
 
 #include "base/memory/scoped_ptr.h"
 #include "base/stl_util.h"
 #include "base/values.h"
 #include "components/content_settings/core/browser/content_settings_utils.h"
@@ skipping 199 matching lines @@
            WebsiteSettingsInfo::UNSYNCABLE, WhitelistedSchemes(),
            ValidSettings(CONTENT_SETTING_ALLOW, CONTENT_SETTING_BLOCK,
                          CONTENT_SETTING_ASK));
 #endif
 
   Register(CONTENT_SETTINGS_TYPE_DURABLE_STORAGE, "durable-storage",
            CONTENT_SETTING_ASK, WebsiteSettingsInfo::UNSYNCABLE,
            WhitelistedSchemes(),
            ValidSettings(CONTENT_SETTING_ALLOW, CONTENT_SETTING_BLOCK));
 
+  Register(CONTENT_SETTINGS_TYPE_USB, "usb", CONTENT_SETTING_ALLOW,
+           WebsiteSettingsInfo::UNSYNCABLE, WhitelistedSchemes(),
+           ValidSettings(CONTENT_SETTING_ALLOW, CONTENT_SETTING_BLOCK));
+
   // Content settings that aren't used to store any data. TODO(raymes): use a
   // different mechanism rather than content settings to represent these.
   // Since nothing is stored in them, there is no real point in them being a
   // content setting.
   Register(CONTENT_SETTINGS_TYPE_PROTOCOL_HANDLERS, "protocol-handler",
            CONTENT_SETTING_DEFAULT, WebsiteSettingsInfo::UNSYNCABLE,
            WhitelistedSchemes(), ValidSettings());
 
   Register(CONTENT_SETTINGS_TYPE_MIXEDSCRIPT, "mixed-script",
            CONTENT_SETTING_DEFAULT, WebsiteSettingsInfo::UNSYNCABLE,
@@ skipping 20 matching lines @@
       website_settings_registry_->Register(type, name, default_value.Pass(),
                                            sync_status,
                                            WebsiteSettingsInfo::NOT_LOSSY);
   DCHECK(!ContainsKey(content_settings_info_, type));
   content_settings_info_.set(
       type, make_scoped_ptr(new ContentSettingsInfo(
                 website_settings_info, whitelisted_schemes, valid_settings)));
 }
 
 }  // namespace content_settings