Make sure ABP category cannot be removed (work-around for AVG toolbar issue)

lib/contentPolicy.js

Index: lib/contentPolicy.js
===================================================================
--- a/lib/contentPolicy.js
+++ b/lib/contentPolicy.js
@@ -368,31 +368,37 @@ let PolicyImplementation =
     registrar.registerFactory(this.classID, this.classDescription, this.contractID, this);
 
     let catMan = Utils.categoryManager;
     for each (let category in this.xpcom_categories)
       catMan.addCategoryEntry(category, this.contractID, this.contractID, false, true);
 
     Services.obs.addObserver(this, "http-on-modify-request", true);
     Services.obs.addObserver(this, "content-document-global-created", true);
+    Services.obs.addObserver(this, "xpcom-category-entry-removed", true);
+    Services.obs.addObserver(this, "xpcom-category-cleared", true);
 
     onShutdown.add(function()
     {
+      // Our category observers should be removed before changing category
+      // memberships, just in case.
+      Services.obs.removeObserver(this, "http-on-modify-request");
+      Services.obs.removeObserver(this, "content-document-global-created");
+      Services.obs.removeObserver(this, "xpcom-category-entry-removed");
+      Services.obs.removeObserver(this, "xpcom-category-cleared");
+
       for each (let category in this.xpcom_categories)
         catMan.deleteCategoryEntry(category, this.contractID, false);
 
       // This needs to run asynchronously, see bug 753687
       Utils.runAsync(function()
       {
         registrar.unregisterFactory(this.classID, this);
       }.bind(this));
 
-      Services.obs.removeObserver(this, "http-on-modify-request");
-      Services.obs.removeObserver(this, "content-document-global-created");
-
       this.previousRequest = null;
     }.bind(this));
   },
 
   //
   // nsISupports interface implementation
   //
 
@@ -491,16 +497,35 @@ let PolicyImplementation =
         {
           let wnd = Utils.getRequestWindow(subject);
           if (wnd && wnd.opener && wnd.location.href == "about:blank")
             this.observe(wnd, "content-document-global-created", null, subject.URI);
         }
 
         break;
       }
+      case "xpcom-category-entry-removed":
+      case "xpcom-category-cleared":
+      {
+        let category = data;
+        if (this.xpcom_categories.indexOf(category) < 0)
+          return;
+
+        if (topic == "xpcom-category-entry-removed" &&
+            subject instanceof Ci.nsISupportsCString &&
+            subject.data != this.contractID)
+        {
+          return;
+        }
+
+        // Our category entry was removed, make sure to add it back
+        let catMan = Utils.categoryManager;
+        catMan.addCategoryEntry(category, this.contractID, this.contractID, false, true);
+        break;
+      }
     }
   },
 
   //
   // nsIChannelEventSink interface implementation
   //
 
   asyncOnChannelRedirect: function(oldChannel, newChannel, flags, callback)