bindings: Set null values on nullable members of CookieListItem

CookieListItem is an IDL dictionary which is used as an output
parameter.
It has two nullable members, but their values were not set if
they were null.

This CL explicitly sets null values if it is expected.

Bug: 839389
Change-Id: I29b4bf81a3db72835617c16f7b3120c1d2be9409
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2269381
Auto-Submit: Hitoshi Yoshida <peria@chromium.org>
Commit-Queue: Ayu Ishii <ayui@chromium.org>
Reviewed-by: Ayu Ishii <ayui@chromium.org>
Cr-Commit-Position: refs/heads/master@{#783037}

third_party/blink/renderer/modules/cookie_store/cookie_change_event.cc

@@ -83,12 +83,18 @@ CookieListItem* CookieChangeEvent::ToCookieListItem(
 
   // The domain of host-only cookies is the host name, without a dot (.) prefix.
   String cookie_domain = canonical_cookie.Domain();
-  if (cookie_domain.StartsWith("."))
+  if (cookie_domain.StartsWith(".")) {
     list_item->setDomain(cookie_domain.Substring(1));
+  } else {
+    list_item->setDomain(String());
+  }
 
   if (!is_deleted) {
     list_item->setValue(canonical_cookie.Value());
-    if (!canonical_cookie.ExpiryDate().is_null()) {
+    if (canonical_cookie.ExpiryDate().is_null()) {
+      // TODO(crbug.com/1070871): Use base::nullopt instead.
+      list_item->setExpiresToNull();
+    } else {
       list_item->setExpires(ConvertSecondsToDOMTimeStamp(
           canonical_cookie.ExpiryDate().ToDoubleT()));
     }