[MD Bookmarks] Add a drag and drop indicator to bookmarks.

chrome/browser/resources/md_bookmarks/item.html

 <link rel="import" href="chrome://resources/html/polymer.html">
 <link rel="import" href="chrome://resources/cr_elements/icons.html">
 <link rel="import" href="chrome://resources/html/icon.html">
 <link rel="import" href="chrome://resources/polymer/v1_0/iron-icon/iron-icon.html">
 <link rel="import" href="chrome://resources/polymer/v1_0/paper-icon-button/paper-icon-button-light.html">
 <link rel="import" href="chrome://bookmarks/actions.html">
 <link rel="import" href="chrome://bookmarks/shared_style.html">
 <link rel="import" href="chrome://bookmarks/store_client.html">
 
 <dom-module id="bookmarks-item">
   <template>
     <style include="shared-style">
       :host {
         -webkit-padding-start: 20px;
         -webkit-user-select: none;
         align-items: center;
         cursor: pointer;
         display: flex;
         flex-direction: row;
         height: 40px;
+        position: relative;
       }
 
       :host([is-selected-item_]) {
         background-color: rgb(225, 235, 253);
       }
 
       #website-title {
         color: var(--primary-text-color);
         flex: 1;
         overflow: hidden;
@@ skipping 35 matching lines @@
     </div>
     <button is="paper-icon-button-light" class="more-vert-button"
         on-click="onMenuButtonOpenClick_">
       <div></div>
       <div></div>
       <div></div>
     </button>
   </template>
   <script src="chrome://bookmarks/item.js"></script>
 <dom-module>