[NTP] Fix the focus ring on gear icon to be square

The reason why the focus ring on gear icon is not square is because the
default setting of the button tag. Now we change edit-bg-gear to be a
div and add a role=button to make sure the a11y behaviour.

Screenshot:
https://screenshot.googleplex.com/TJpu8fFFpLW.png

Bug: 867777
Change-Id: Ifb8938b1fed139b3ba821976388bf154a58e8d85
Reviewed-on: https://chromium-review.googlesource.com/1176600Reviewed-by: Kristi Park <kristipark@chromium.org>
Commit-Queue: Weilun Shi <sweilun@chromium.org>
Cr-Commit-Position: refs/heads/master@{#583764}

chrome/browser/resources/local_ntp/local_ntp.html

@@ -79,7 +79,7 @@
     <div id="attribution"><div id="attribution-text"></div></div>
 
     <div id="edit-bg" tabindex="0" hidden>
-      <button id="edit-bg-gear" tabindex="-1"></button>
+      <div id="edit-bg-gear" tabindex="-1" role="button"></div>
     </div>
 
     <div id="message-box-container" class="message-box-hide">