Add ability to fetch and run recipes early on firstrun

[Osmose]

We'd like to add a feature to Normandy that triggers fetching and executing recipes as early on a user's first run of Firefox as we can reasonably get.

• Once all recipes have finished executing, we should write the time since launch to telemetry so that we can analyze how reliably we can modify things like preferences at firstrun. TBD on how to structure this (a custom one-time packet? on the main ping? data review?).
  • Thinking more, we probably want to include individual recipe IDs and their finish times so we can measure a specific preference change, for example.
• We should add a field to filter expressions to allow filtering recipes to only execute on firstrun (normandy.isFirstRun?)

[rhelmer]

I think that the earliest we can reasonably start running something like fetching code is after sessionstore-windows-restored (see https://developer.mozilla.org/en-US/docs/Observer_Notifications) - we'll need an observer like:

Cu.import("resource://gre/modules/XPCOMUtils.jsm");
Cu.import("resource://gre/modules/Services.jsm");

let observer = {
  observe: function(subject, topic, data)
  {
    Services.obs.removeObserver("sessionstore-windows-restored", this);
    // fetch and execute recipes
  },
  QueryInterface: XPCOMUtils.generateQI([
    Components.interfaces.nsIObserver,
    Components.interfaces.nsISupportsWeakReference
  ])
};
Services.obs.addObserver("sessionstore-windows-restored", observer, true);

Since there's already an update timer being used I suspect we can just trigger it to run when the observer notification fires.

[rhelmer]

OK. So it looks like RecipeRunner.jsm registers a timer with the ID recipe-client-addon-run, which fetches and executes recipes.

What we should be able to do here is add a "sessionstore-windows-restored" observer early in the startup of the system add-on, which calls notify() on the timer.

[rhelmer]

On further reflection I think it's simpler and more readable to just run once and set the timer up after "sessionstore-windows-restored" is observed, rather than let the timer be set up and notify it explicitly etc.

[Osmose]

#895 fixed this and it got uplifted and is currently in release.