[NTP] Prevent the Cancel and Delete buttons from submitting the edit custom link form

<button> is type="submit" by default. Adding type="button" will prevent
the Cancel and Delete buttons from triggering the "submit" event.

Bug: 874018
Change-Id: Ie72ec07ddbaeccee29588612fc8938f268150f20
Reviewed-on: https://chromium-review.googlesource.com/1174804
Commit-Queue: Kristi Park <kristipark@chromium.org>
Reviewed-by: Fernando Serboncini <fserb@chromium.org>
Cr-Commit-Position: refs/heads/master@{#583441}

chrome/browser/resources/local_ntp/custom_links_edit.html

@@ -31,9 +31,9 @@
         <div id="invalid-url" class="error-msg"></div>
       </div>
       <div class="buttons-container">
-        <button id="delete" class="secondary" tabindex="0"></button>
+        <button id="delete" class="secondary" type="button" tabindex="0"></button>
         <span class="right">
-          <button id="cancel" class="secondary" tabindex="0"></button>
+          <button id="cancel" class="secondary" type="button" tabindex="0"></button>
           <button type="submit" id="done" class="primary" tabindex="0"></button>
         </span>
       </div>