Bug 1224577: Fix removing frames from the map so chrome windows aren't leaked from every add-on. r=bz

--HG--
extra : commitid : 5ULh3d5M5aV
extra : rebase_source : 310922f621d4099711637ee94134435aa4769a22

addon-sdk/source/lib/sdk/remote/parent.js

@@ -194,9 +194,9 @@ const Frame = Class({
   dispose: function() {
     emit(this, 'detach', this);
     ns(this).messageManager.removeMessageListener('sdk/remote/frame/message', ns(this).messageReceived);
-    ns(this).messageManager = null;
 
     frameMap.delete(ns(this).messageManager);
+    ns(this).messageManager = null;
   },
 
   // Returns the browser or iframe element this frame displays in