Handle multiple unique identifiers for localization
<rdar://problem/114840127>
It turns out this is not needed, and Safari was doing this incorrectly. According to the Chrome docs, the special @@extension_id message can only be used in CSS files, not the manifest. So this bug isn't a real problem. "The extension or app ID; you might use this string to construct URLs for resources inside the extension. Even unlocalized extensions can use this message. Note: You can't use this message in a manifest file." https://developer.chrome.com/docs/extensions/reference/i18n/#overview-predefined