Call document.close, triggering onload and making the test not time out.

This fixes both uses of object-association.js.

bugzilla-url: https://bugzilla.mozilla.org/show_bug.cgi?id=1419297
gecko-commit: 2762fef0119bcd3e78d36e78b0b75b5448d35c85
gecko-integration-branch: autoland
gecko-reviewers: smaug

common/object-association.js

@@ -14,7 +14,7 @@ window.testIsPerWindow = propertyName => {
     iframe.remove();
 
     const after = frame[propertyName];
-    assert_equals(after, before);
+    assert_equals(after, before, `window.${propertyName} should not change after iframe.remove()`);
   }, `Discarding the browsing context must not change window.${propertyName}`);
 
   async_test(t => {
@@ -56,6 +56,8 @@ window.testIsPerWindow = propertyName => {
 
       const after = frame[propertyName];
       assert_not_equals(after, before);
+
+      frame.document.close();
     });
 
     iframe.src = "/common/blank.html";