This moves AboutNewTab.init from nsBrowserGlue.js handling of "browser-delayed-startup-finished" into aboutNewTabService.js so that when the service is loaded once from the main thread probably by browser.js towards the beginning of _delayedStartup just before potentially calling gBrowser.loadTabs, the service triggers the attaching of RemotePages(about:newtab) before any about:newtab pages load.
Additionally even when RemotePages starts early enough, Activity Stream might not borrow the RemotePages instance early enough to catch the RemotePage:Load message, so to simulate that, RemotePages now remembers when a port has been loaded for consumers to check. Adds tests to confirm the expected properties on the port and value of loaded at the various RemotePage:* messages.
MozReview-Commit-ID: IXJLvFCgbEH
