MD Settings: Remove paper-header-panel, use IntersectionObserver.

chrome/browser/resources/settings/settings_page/main_page_behavior.js

 // Copyright 2016 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.
 
 /**
  * Responds to route changes by expanding, collapsing, or scrolling to sections
  * on the page. Expanded sections take up the full height of the container. At
  * most one section should be expanded at any given time.
  * @polymerBehavior MainPageBehavior
  */
@@ skipping 23 matching lines @@
 
   /** @type {?HTMLElement} The scrolling container. */
   scroller: null,
 
   listeners: {
     'neon-animation-finish': 'onNeonAnimationFinish_'
   },
 
   /** @override */
   attached: function() {
-    if (this.domHost && this.domHost.parentNode.tagName == 'PAPER-HEADER-PANEL')
-      this.scroller = this.domHost.parentNode.scroller;
-    else
-      this.scroller = document.body; // Used in unit tests.
+    this.scroller = this.domHost ? this.domHost.parentNode : document.body;
   },
 
   /**
    * Remove the is-animating attribute once the animation is complete.
    * This may catch animations finishing more often than needed, which is not
    * known to cause any issues (e.g. when animating from a shallower page to a
    * deeper page; or when transitioning to the main page).
    * @private
    */
   onNeonAnimationFinish_: function() {
@@ skipping 289 matching lines @@
     return /** @type {?SettingsSectionElement} */(
         this.$$('settings-section[section="' + section + '"]'));
   },
 };
 
 /** @polymerBehavior */
 var MainPageBehavior = [
   settings.RouteObserverBehavior,
   MainPageBehaviorImpl,
 ];