Allows you to specify which URLs are allowed to install extensions, apps, and themes.
Starting in Google Chrome 21, it is more difficult to install extensions, apps, and user scripts from outside the Chrome Web Store. Previously, users could click on a link to a *.crx file, and Google Chrome would offer to install the file after a few warnings. After Google Chrome 21, such files must be downloaded and dragged onto the Google Chrome settings page. This setting allows specific URLs to have the old, easier installation flow.
Each item in this list is an extension-style match pattern (see https://developer.chrome.com/extensions/match_patterns). Users will be able to easily install items from any URL that matches an item in this list. Both the location of the *.crx file and the page where the download is started from (i.e. the referrer) must be allowed by these patterns.
ExtensionInstallBlacklist takes precedence over this policy. That is, an extension on the blacklist won't be installed, even if it happens from a site on this list.