MD Settings: long dialog body should have overscroll line.

ui/webui/resources/cr_elements/cr_dialog/cr_dialog.html

 <link rel="import" href="chrome://resources/cr_elements/icons.html">
 <link rel="import" href="chrome://resources/html/assert.html">
 <link rel="import" href="chrome://resources/html/polymer.html">
 <link rel="import" href="chrome://resources/polymer/v1_0/paper-icon-button/paper-icon-button.html">
 <link rel="import" href="chrome://resources/polymer/v1_0/paper-styles/default-theme.html">
 
 <dom-module id="cr-dialog">
   <template>
     <style>
       :host {
@@ skipping 41 matching lines @@
         };
         -webkit-margin-end: 6px;
         /* <paper-icon-button> overrides --iron-icon-{height,width}, so this
          * padding essentially reduces 40x40 to 20x20. */
         padding: 10px;
       }
 
       .body-container {
         display: flex;
         flex-direction: column;
-        /* TODO(dbeam): copy <paper-dialog-scrollable>'s dividers? */
         overflow: auto;
+        @apply(--cr-dialog-body-container);
+      }
+
+      :host([show-scroll-borders]) .body-container {
+        /* Prevent layout moving when border does appear. */
+        border-bottom: 1px solid transparent;
+      }
+
+      :host([show-scroll-borders]) .body-container.bottom-scrollable {
+        border-bottom: 1px solid var(--paper-grey-300);
       }
 
       :host ::content .body {
         padding-bottom: 12px;
         padding-top: 12px;
       }
 
       :host ::content .body,
       :host ::content .title {
         -webkit-padding-end: 24px;
@@ skipping 29 matching lines @@
     <div class="top-container">
       <div class="title-container" tabindex="-1">
         <content select=".title"></content>
       </div>
       <paper-icon-button icon="cr:clear" on-tap="cancel" id="close"
           aria-label$="[[closeText]]">
       </paper-icon-button>
     </div>
     <div class="body-container">
       <content select=".body"></content>
+      <span id="bodyBottomMarker"></span>
     </div>
     <content select=".button-container"></content>
     <content select=".footer"></content>
   </template>
   <script src="cr_dialog.js"></script>
 </dom-module>