Update comment in modulepreload web platform test

Bug: 740886
Change-Id: I7cdfd6099a24bd36ccb1f962317f83b5e5299f20
Reviewed-on: https://chromium-review.googlesource.com/828342Reviewed-by: Kouhei Ueno <kouhei@chromium.org>
Commit-Queue: Kunihiko Sakamoto <ksakamoto@chromium.org>
Cr-Commit-Position: refs/heads/master@{#524645}

third_party/WebKit/LayoutTests/external/wpt/preload/modulepreload.html

@@ -53,8 +53,8 @@ promise_test(function(t) {
     link.rel = 'modulepreload';
     link.href = 'resources/module1.js';
     return attachAndWaitForLoad(link).then(() => {
-        // Currently, modulepreload doesn't perform submodules fetch.
         verifyNumberOfDownloads('resources/module1.js', 1);
+        // The load event fires before (optional) submodules fetch.
         verifyNumberOfDownloads('resources/module2.js', 0);
 
         var script = document.createElement('script');