[Chrome Colors] Prevent scroll when focusing on first tile.

Bug: 997143
Change-Id: I2c2a7511e79d5fb0fbc72f8892a8541baf9c158c
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1772444Reviewed-by: Kyle Milka <kmilka@chromium.org>
Commit-Queue: Kyle Milka <kmilka@chromium.org>
Cr-Commit-Position: refs/heads/master@{#690755}

chrome/browser/resources/local_ntp/customize.js

@@ -2279,6 +2279,7 @@ customize.loadColorsMenu = function() {
       } else {
         $(customize.IDS.COLORS_DEFAULT_ICON).focus();
       }
+      event.preventDefault();
     }
   };