Settings: Implement sub-subpages, and clean up some animations.

chrome/browser/resources/settings/settings_page/settings_section.css

 /* Copyright 2015 The Chromium Authors. All rights reserved.
  * Use of this source code is governed by a BSD-style license that can be
  * found in the LICENSE file. */
 
 /**
  * @fileoverview
  * Common styles for Settings pages.
  */
 #section {
   @apply(--shadow-elevation-4dp);
   background-color: white;
   height: 100%;
   margin-bottom: 16px;
+  overflow: hidden;
   width: 605px;
 }
 
 #header {
   @apply(--layout-horizontal);
   @apply(--layout-center);
   background-color: #f8f8f8;
   border-bottom: 1px solid rgba(0, 0, 0, .1);
   padding: 16px;
 }
 
 #header iron-icon {
   -webkit-padding-end: 16px;
   color: rgba(0, 0, 0, .87);
 }
 
 #header .title {
   @apply(--paper-font-body2);
 }
 
 #content-container {
   padding: 16px;
 }