WebUI NTP: customize dialog, reset scroll when changing pages

Change-Id: I5215174f057e1e377216db73b7f0bf7a77086f4c
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2079465
Commit-Queue: Esmael Elmoslimany <aee@chromium.org>
Commit-Queue: Tibor Goldschwendt <tiborg@chromium.org>
Auto-Submit: Esmael Elmoslimany <aee@chromium.org>
Reviewed-by: Tibor Goldschwendt <tiborg@chromium.org>
Cr-Commit-Position: refs/heads/master@{#745579}

chrome/browser/resources/new_tab_page/customize_dialog.js

@@ -36,6 +36,7 @@ class CustomizeDialogElement extends PolymerElement {
       selectedPage_: {
         type: String,
         value: 'backgrounds',
+        observer: 'onSelectedPageChange_',
       },
     };
   }
@@ -114,6 +115,11 @@ class CustomizeDialogElement extends PolymerElement {
     e.stopPropagation();
     this.selectedPage_ = e.target.getAttribute('page-name');
   }
+
+  /** @private */
+  onSelectedPageChange_() {
+    this.$.pages.scrollTop = 0;
+  }
 }
 
 customElements.define(CustomizeDialogElement.is, CustomizeDialogElement);