[MD Bookmarks] Allow left/right keys to close/open folders in sidebar.

chrome/browser/resources/md_bookmarks/folder_node.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/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/icons.html">
 <link rel="import" href="chrome://bookmarks/shared_style.html">
 <link rel="import" href="chrome://bookmarks/store_client.html">
 
 <dom-module id="bookmarks-folder-node">
@@ skipping 39 matching lines @@
         color: var(--interactive-color);
       }
 
       iron-icon {
         flex-shrink: 0;
       }
 
       #arrow {
         color: var(--secondary-text-color);
         margin: 0 8px;
+      }
+
+      #arrow-icon {
         transform: rotate(-90deg);
         transition: transform 150ms;
       }
 
       :host-context([dir='rtl']) #arrow {

[tsergeant, 2017/05/05 01:31:07]

This appears to work, but it should probably apply to #arrow-icon instead.

Also, you could apply all these styles to `#arrow iron-icon` if you want to
avoid adding a new ID. Up to you.

[calamity, 2017/05/09 03:45:35]

Done.

         transform: rotate(90deg);
       }
 
-      #arrow[is-open] {
+      #arrow-icon[is-open] {
         transform: initial;
       }
 
       [no-children] {
         -webkit-padding-start: 52px; /* The width of the arrow and its margin */
       }
     </style>
 
     <div id="container" class="v-centered" on-tap="selectFolder_"
         draggable="[[!isTopLevelFolder_(depth)]]"
         tabindex$="[[getTabIndex_(isSelectedFolder_)]]"
         hidden="[[isRootFolder_(depth)]]">
       <template is="dom-if" if="[[hasChildFolder_(item_.children)]]">
         <button is="paper-icon-button-light" id="arrow"
             is-open$="[[!isClosed_]]" on-tap="toggleFolder_"
             on-mousedown="preventDefault_" tabindex="-1">
-          <iron-icon icon="cr:arrow-drop-down"></iron-icon>
+          <iron-icon id="arrow-icon" icon="cr:arrow-drop-down"
+              is-open$="[[!isClosed_]]">
+          </iron-icon>
         </button>
       </template>
       <div id="folder-label" class="v-centered">
         <iron-icon icon="[[getFolderIcon_(isSelectedFolder_)]]"
             no-children$="[[!hasChildFolder_(item_.children)]]">
         </iron-icon>
         <div class="menu-label">[[item_.title]]</div>
       </div>
     </div>
     <div id="descendants" hidden$="[[isClosed_]]">
       <template is="dom-repeat"
           items="[[item_.children]]"
           as="child"
           filter="isFolder_">
         <bookmarks-folder-node item-id="[[child]]"
             depth="[[getChildDepth_(depth)]]">
         </bookmarks-folder-node>
       </template>
     </div>
   </template>
   <script src="chrome://bookmarks/folder_node.js"></script>
 </dom-module>