[webui][ntp] Adjust height and margin of "Customize this page" title

* Adjust height of the title from 58px to 64px.
* Add a 16px top margin to the title.

Bug: 1077458
Change-Id: I1c82831e873432f206379bebef11cf604ce3eaac
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2335497Reviewed-by: Tibor Goldschwendt <tiborg@chromium.org>
Commit-Queue: Yue Zhang <yuezhanggg@chromium.org>
Cr-Commit-Position: refs/heads/master@{#794232}

chrome/browser/resources/new_tab_page/customize_dialog.html

@@ -18,7 +18,8 @@
     color: var(--ntp-primary-text-color);
     display: flex;
     flex-direction: row;
-    height: 58px;
+    height: 64px;
+    margin-top: 16px;
     padding: 0;
   }