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

components/content_settings/core/browser/content_settings_registry.cc

Index: components/content_settings/core/browser/content_settings_registry.cc
diff --git a/components/content_settings/core/browser/content_settings_registry.cc b/components/content_settings/core/browser/content_settings_registry.cc
index faa5f8388c855d829af4c95258b45e01e80b4d03..bc5a8e13dca573ab7b2ee639c87b247ef2a7fca1 100644
--- a/components/content_settings/core/browser/content_settings_registry.cc
+++ b/components/content_settings/core/browser/content_settings_registry.cc
@@ -217,6 +217,10 @@ void ContentSettingsRegistry::Init() {
            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