Bug 1246244 - Follow-up - clean up MockFilePicker once test is done. r=me

MozReview-Commit-ID: 54rjvxSD9ZT

--HG--
extra : rebase_source : a6eb88f3b085dd12a836190a8c0201b33701dc8a

toolkit/content/tests/browser/browser_saveImageURL.js

@@ -6,6 +6,10 @@ const PREF_UNSAFE_FORBIDDEN = "dom.ipc.cpows.forbid-unsafe-from-browser";
 MockFilePicker.init(window);
 MockFilePicker.returnValue = MockFilePicker.returnCancel;
 
+registerCleanupFunction(function() {
+  MockFilePicker.cleanup();
+});
+
 function waitForFilePicker() {
   return new Promise((resolve) => {
     MockFilePicker.showCallback = () => {