This removes all docshell nsISecureBrowserUI and mixed content properties, and moves them into CanonicalBrowsingContext/WindowGlobalParent. It makes the mixed content blocker just compute the state for the current load, and then send the results to the parent process, where we update the security state accordingly.
I think we could in the future remove onSecurityChange entirely, and instead just fire an event to the <browser> element notifying it of changes to the queryable securityUI.
Unfortunately we have a lot of existing code that depends on specific ordering between onSecurityChange and onLocationChange, so I had to hook into the RemoteWebProgress implementation in BrowserParent to mimic the same timings.
Differential Revision: https://phabricator.services.mozilla.com/D75447
e060a86c42