First reported by Dayton Lowell, the message briefly appears at http://icloud.com/bookmarks_extension when trying to install the Chrome extension from the iCloud Control Panel for Windows.
It's thought that perhaps the message indicates that users will eventually be able to view their iCloud synced bookmarks on the web. This would be especially useful to people who don't have immediate access to one of their own devices and need to view a bookmark.
The message also makes use of the iOS 7 icon for Bookmarks which indicates the page is somewhat recent. Apple launched its browser extensions alongside iOS 7 so there doesn't appear to be a need for a 'coming soon' page if the feature was extension related.
Take a look at a screenshot below. You can follow iClarified on Twitter, Facebook, Google+, or RSS for details on any further discoveries.