[MD History] Add shadow to toolbar on scroll.

chrome/browser/resources/md_history/app.html

 <link rel="import" href="chrome://resources/html/polymer.html">
 <link rel="import" href="chrome://resources/html/cr/ui.html">
 <link rel="import" href="chrome://resources/html/cr/ui/command.html">
 <link rel="import" href="chrome://resources/polymer/v1_0/app-layout/app-drawer/app-drawer.html">
 <link rel="import" href="chrome://resources/polymer/v1_0/app-route/app-location.html">
 <link rel="import" href="chrome://resources/polymer/v1_0/app-route/app-route.html">
 <link rel="import" href="chrome://resources/polymer/v1_0/iron-media-query/iron-media-query.html">
 <link rel="import" href="chrome://resources/polymer/v1_0/iron-pages/iron-pages.html">
+<link rel="import" href="chrome://resources/polymer/v1_0/iron-scroll-target-behavior/iron-scroll-target-behavior.html">
 <link rel="import" href="chrome://history/history_toolbar.html">
 <link rel="import" href="chrome://history/list_container.html">
 <link rel="import" href="chrome://history/synced_device_manager.html">
 <link rel="import" href="chrome://history/side_bar.html">
 <link rel="import" href="chrome://history/shared_style.html">
 <link rel="stylesheet" href="chrome://resources/css/md_colors.css">
 
 <dom-module id="history-app">
   <template>
     <style no-process>
       history-toolbar {
         background: var(--md-toolbar-color);
       }
     </style>
     <style include="shared-style">
       :host {
         display: block;
         height: 100%;
         overflow: hidden;
       }
 
       /* Sizing this with flex causes slow load performance, see
        * crbug.com/618153. TODO(dbeam): is this still an issue? */
       #main-container {
         height: calc(100% - var(--toolbar-height));
+        position: relative;
       }
 
       :host([grouped_]) #main-container {
         height: calc(100% - var(--toolbar-grouped-height));
       }
 
       #content-side-bar {
         float: left;
       }
 
@@ skipping 13 matching lines @@
         height: var(--toolbar-height);
         margin-bottom: 5px;
       }
 
       h1 {
         -webkit-padding-start: 24px;
         color: rgb(66, 66, 66);
         font-size: 123%;
         font-weight: 400;
       }
+
+      #drop-shadow {
+        box-shadow: inset 0 5px 6px -3px rgba(0, 0, 0, 0.4);
+        height: 6px;
+        left: 0;
+        opacity: 0;
+        pointer-events: none;
+        position: absolute;
+        right: 0;
+        top: 0;
+        transition: opacity 500ms;
+      }
+
+      :host([toolbar-shadow_]) #drop-shadow {
+        opacity: 1;
+      }
     </style>
     <app-location route="{{route_}}"></app-location>
     <app-route route="{{route_}}" pattern="/:page" data="{{routeData_}}"
         query-params="{{queryParams_}}">
     </app-route>
     <history-toolbar id="toolbar"
         spinner-active="[[shouldShowSpinner_(queryState_.querying,
                                              queryState_.incremental,
                                              queryState_.searchTerm)]]"
         is-grouped-mode="{{grouped_}}"
@@ skipping 19 matching lines @@
         </history-list-container>
         <template is="dom-if" if="[[syncedTabsSelected_(selectedPage_)]]">
           <history-synced-device-manager id="synced-devices"
               session-list="[[queryResult_.sessionList]]"
               search-term="[[queryState_.searchTerm]]"
               sign-in-state="[[isUserSignedIn_]]"
               path="syncedTabs">
           </history-synced-device-manager>
         </template>
       </iron-pages>
+      <div id="drop-shadow"></div>
     </div>
 
     <template is="dom-if" if="[[hasDrawer_]]">
       <app-drawer id="drawer" swipe-open align="start" tabindex="0">
         <div id="drawer-header">
           <h1>$i18n{title}</h1>
         </div>
         <history-side-bar id="drawer-side-bar" selected-page="[[selectedPage_]]"
             route="[[route_]]" show-footer="[[showSidebarFooter]]" drawer>
         </history-side-bar>
       </app-drawer>
     </template>
 
     <iron-media-query query="(max-width: 1023px)"
         query-matches="{{hasDrawer_}}">
     </iron-media-query>
   </template>
   <script src="chrome://history/app.js"></script>
 </dom-module>