Navi: Fix wallpaper module keyboard focus

Bug: 955357
Change-Id: I50ecd9e1a2f6962fb3e8fe9f67627e13e2c09983
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1579024
Auto-Submit: Hector Carmona <hcarmona@chromium.org>
Commit-Queue: Dan Beam <dbeam@chromium.org>
Reviewed-by: Dan Beam <dbeam@chromium.org>
Cr-Commit-Position: refs/heads/master@{#653611}

chrome/browser/resources/welcome/onboarding_welcome/ntp_background/nux_ntp_background.js

@@ -180,7 +180,7 @@ Polymer({
       direction *= -1;  // Reverse direction if RTL.
     }
 
-    const buttons = this.root.querySelectorAll('.ntp-background-grid-button');
+    const buttons = this.root.querySelectorAll('.option');
     const targetIndex = Array.prototype.indexOf.call(buttons, element);
 
     const oldFocus = buttons[targetIndex];