Fixes NTP Modules not showing if there is no promo

Modules wait for the promo to be loaded before showing. The promo is
considered loaded when the ntp-middle-slot-promo has been fired by the
promo element. This event is expected to be fired whether or not there
is a promo data available. crrev.com/c/2505739 changed that behavior
causing NTP Modules to remain hidden when there is no promo data.

Fixed: 1147257, 1143366
Change-Id: If0c14ec8ee142fd38cdbc1bf9249f3345db6ca82
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2527854
Commit-Queue: Moe Ahmadi <mahmadi@chromium.org>
Commit-Queue: Tibor Goldschwendt <tiborg@chromium.org>
Auto-Submit: Moe Ahmadi <mahmadi@chromium.org>
Reviewed-by: Tibor Goldschwendt <tiborg@chromium.org>
Cr-Commit-Position: refs/heads/master@{#825668}

chrome/browser/resources/new_tab_page/middle_slot_promo.js

@@ -102,6 +102,11 @@ class MiddleSlotPromoElement extends PolymerElement {
           this.dispatchEvent(new Event(
               'ntp-middle-slot-promo-loaded', {bubbles: true, composed: true}));
         });
+      } else {
+        // Dispatch this event even if there is no promo as Modules wait for the
+        // promo to be loaded before showing.
+        this.dispatchEvent(new Event(
+            'ntp-middle-slot-promo-loaded', {bubbles: true, composed: true}));
       }
     });
   }

chrome/test/data/webui/new_tab_page/middle_slot_promo_test.js

@@ -144,11 +144,14 @@ suite('NewTabPageMiddleSlotPromoTest', () => {
     testProxy.handler.setResultFor('getPromo', Promise.resolve({promo: null}));
     const middleSlotPromo = document.createElement('ntp-middle-slot-promo');
     document.body.appendChild(middleSlotPromo);
+    const loaded =
+        eventToPromise('ntp-middle-slot-promo-loaded', document.body);
     assertEquals(
         0,
         promoBrowserCommandTestProxy.handler.getCallCount(
             'canShowPromoWithCommand'));
     assertEquals(0, testProxy.handler.getCallCount('onPromoRendered'));
     assertTrue(middleSlotPromo.hasAttribute('hidden'));
+    await loaded;
   });
 });