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

chrome/browser/resources/settings/settings_ui/settings_ui.js

 // 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
  * 'settings-ui' implements the UI for the Settings page.
  *
  * Example:
  *
@@ skipping 142 matching lines @@
           googleDrive: false,
         },
 // </if>
       };
     }
 
     this.showAndroidApps_ = loadTimeData.valueExists('androidAppsAllowed') &&
         loadTimeData.getBoolean('androidAppsAllowed');
   },
 
+  /** @private {?IntersectionObserver} */
+  intersectionObserver_: null,
+
   /** @override */
   attached: function() {
     setTimeout(function() {
       chrome.send(
           'metricsHandler:recordTime',
           ['Settings.TimeUntilInteractive', window.performance.now()]);
     });
     // Preload bold Roboto so it doesn't load and flicker the first time used.
     document.fonts.load('bold 12px Roboto');
-    settings.setGlobalScrollTarget(this.$.headerPanel.scroller);
+    settings.setGlobalScrollTarget(this.$.container);
+
+    // Setup drop shadow logic.
+    var callback = function(entries) {
+      assert(entries.length == 1);
+      this.$.dropShadow.classList.toggle(
+          'has-shadow', entries[0].intersectionRatio == 0);
+    }.bind(this);
+
+    this.intersectionObserver_ = new IntersectionObserver(
+        callback,
+        /** @type {IntersectionObserverInit} */ ({
+          root: this.$.container,
+          threshold: 0,
+        }));
+    this.intersectionObserver_.observe(this.$.intersectionProbe);
   },
 
   /** @override */
   detached: function() {
     settings.resetRouteForTesting();
+    this.intersectionObserver_.disconnect();
+    this.intersectionObserver_ = null;
   },
 
   /** @param {!settings.Route} route */
   currentRouteChanged: function(route) {
     var urlSearchQuery = settings.getQueryParameters().get('search') || '';
     if (urlSearchQuery == this.lastSearchQuery_)
       return;
 
     this.lastSearchQuery_ = urlSearchQuery;
 
@@ skipping 54 matching lines @@
   onMenuButtonTap_: function() {
     this.$.drawer.toggle();
   },
 
   /** @private */
   directionDelegateChanged_: function() {
     this.$.drawer.align = this.directionDelegate.isRtl() ?
         'right' : 'left';
   },
 });