[webui][ntp] Ensure MV titles of 3P themes are dark

Previously, the titles honored the theme setting. However, the title is
shown in a white pill and, therefore, was invisible if the theme set a
light color. Thus, this CL always sets the title color to dark. This is
also what the local NTP does.

Before: https://screenshot.googleplex.com/3W85KAw7sbv
After: https://screenshot.googleplex.com/d2pAxBWjRx9

Fixed: 1105941
Change-Id: I4ca38bd3e70cf0e669ea2f5d23f6821ef822bf43
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2300662
Commit-Queue: Tibor Goldschwendt <tiborg@chromium.org>
Commit-Queue: Moe Ahmadi <mahmadi@chromium.org>
Auto-Submit: Tibor Goldschwendt <tiborg@chromium.org>
Reviewed-by: Moe Ahmadi <mahmadi@chromium.org>
Cr-Commit-Position: refs/heads/master@{#788827}

chrome/browser/resources/new_tab_page/most_visited.html

@@ -110,6 +110,7 @@
 
   :host([use-title-pill]) .tile-title {
     background-color: white;
+    color: var(--google-grey-800);
   }
 
   .tile-title span {

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

@@ -769,6 +769,7 @@ suite('NewTabPageMostVisitedTest', () => {
     });
     queryAll('.tile-title span').forEach(tile => {
       assertStyle(tile, 'text-shadow', 'none');
+      assertStyle(tile, 'color', 'rgb(60, 64, 67)');
     });
   });