[NTP] Remove 'white flash' transition from MV tiles

Use smoother transition when the MV tiles are updated. This removes the
'white flash' that is currently present.

Screencast: https://screencast.googleplex.com/cast/NTM0NjkzNjMyNDM1ODE0NHxhZWM4ZWEyOS04MA

Bug: 905119
Change-Id: Ic1a7ea35b2e05bbcbcd583b2ca792a6b7c8f2dc0
Reviewed-on: https://chromium-review.googlesource.com/c/1334938Reviewed-by: Ramya Nagarajan <ramyan@chromium.org>
Commit-Queue: Kristi Park <kristipark@chromium.org>
Cr-Commit-Position: refs/heads/master@{#608071}

chrome/browser/resources/local_ntp/most_visited_single.css

@@ -80,7 +80,8 @@ a:visited {
   user-select: none;
 }
 
-.md #mv-tiles {
+.md #mv-tiles,
+.md .mv-tiles-old {
   display: flex;
   flex-wrap: wrap;
   height: auto;
@@ -91,10 +92,14 @@ a:visited {
    * values in local_ntp.css. */
   max-width: calc(var(--md-tile-width) * var(--md-max-tiles-row));
   position: static;
+  transition-duration: 300ms;
 }
 
 .md .mv-tiles-old {
-  display: none;
+  left: 0;
+  margin: auto;
+  position: absolute;
+  right: 0;
 }
 
 .mv-tile,