browser updater failure (self.update is undefined)

In testing the updater using the candidate 4.0a2 builds, brade and I found a problem that causes the update to silently fail. On the browser console, we see:

[15:20:56.361] WARN addons.manager: Exception calling callback:
  TypeError: self.update is undefined @ chrome://browser/content/aboutDialog.js:439

It looks like we used self.update in a couple of functions where we should have used this.update. I am not sure why these code paths were not exercised in our earlier testing, but we are working on a fix right now.

If it is not too late, we should respin 4.0a2 to pick up this fix.