Some security checks look at extension URLs and want to distinguish between extensions that have been uninstalled and those that were never installed.  This matters because some aspects of an extension can stick around and continue to make requests after uninstallation, such as XHRs from content scripts or extension subframes.

If we keep track of which extensions have been uninstalled, Chrome can avoid treating these leftover actions as malicious.  This might help avoid unnecessary renderer kills, as we saw in issue 613335.

One example of a check that could benefit from this list is CheckOriginHeader in ChromeContentBrowserClientExtensionsPart.  In that case, it would be worth tracking not just which extensions were uninstalled, but also whether they were platform apps.