[NTP] Fix z-index for MV tiles

Relative is required for z-index to function. This will place the icon
and title behind the semi-transparent background on hover.
Screenshot: https://screenshot.googleplex.com/AK4t7nhOJfr.png

Bug: 872206, 864357
Change-Id: I2300cd6f5ef3a012ef7be2501f8cf27bc4d6f220
Reviewed-on: https://chromium-review.googlesource.com/1167648
Commit-Queue: Kristi Park <kristipark@chromium.org>
Reviewed-by: Mathieu Perreault <mathp@chromium.org>
Cr-Commit-Position: refs/heads/master@{#581744}

chrome/browser/resources/local_ntp/most_visited_single.css

@@ -433,6 +433,7 @@ body.dark-theme .md-tile-container:active + .md-menu::after {
   display: flex;
   flex-flow: column nowrap;
   height: 100%;
+  position: relative;
   width: 100%;
   z-index: -1;
 }