[NTP] Re-add the opacity transition for shortcuts on first load

The transition was removed by changes in https://crrev.com/c/1591954.

Technically, the transition is a regression from pre-M69 behavior, but
is now considered accepted.

Change-Id: I832263f37f29b01c8c8e8bd699240ed21431bc6b
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1604426
Commit-Queue: Kristi Park <kristipark@chromium.org>
Reviewed-by: Kyle Milka <kmilka@chromium.org>
Cr-Commit-Position: refs/heads/master@{#658763}

chrome/browser/resources/local_ntp/most_visited_single.js

@@ -832,11 +832,8 @@ function swapInNewTiles() {
 
   const parent = document.querySelector('#' + IDS.MOST_VISITED);
 
-  // Only fade in the new tiles if there were tiles before.
-  let fadeIn = false;
   const old = parent.querySelector('#' + IDS.MV_TILES);
   if (old) {
-    fadeIn = true;
     // Mark old tile DIV for removal after the transition animation is done.
     old.removeAttribute('id');
     old.classList.add('mv-tiles-old');
@@ -860,9 +857,7 @@ function swapInNewTiles() {
 
   // getComputedStyle causes the initial style (opacity 0) to be applied, so
   // that when we then set it to 1, that triggers the CSS transition.
-  if (fadeIn) {
-    flushOpacity();
-  }
+  flushOpacity();
   cur.style.opacity = 1.0;
 
   if (document.documentElement.classList.contains(CLASSES.NO_INITIAL_FADE)) {