Bug 1213990 Do immediate uninstall for test webextensions r=kmag

MozReview-Commit-ID: 9vcAfPhRYFi

--HG--
extra : rebase_source : c10b7e927eb0d0e12f70d9e06f77d1b56fa405fc

toolkit/components/extensions/Extension.jsm

@@ -1173,7 +1173,7 @@ class MockExtension {
   }
 
   shutdown() {
-    this.addon.uninstall(true);
+    this.addon.uninstall();
     return this.cleanupGeneratedFile();
   }