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

chrome/browser/resources/md_bookmarks/folder_node.html

Index: chrome/browser/resources/md_bookmarks/folder_node.html
diff --git a/chrome/browser/resources/md_bookmarks/folder_node.html b/chrome/browser/resources/md_bookmarks/folder_node.html
index 9f87024a58dc4b0c06d1d40411bceec2ee94c550..42c85279c25f3e7b4ddd13df40bb9a002f34c8ba 100644
--- a/chrome/browser/resources/md_bookmarks/folder_node.html
+++ b/chrome/browser/resources/md_bookmarks/folder_node.html
@@ -57,6 +57,9 @@
       #arrow {
         color: var(--secondary-text-color);
         margin: 0 8px;
+      }
+
+      #arrow-icon {
         transform: rotate(-90deg);
         transition: transform 150ms;
       }
@@ -65,7 +68,7 @@
         transform: rotate(90deg);
       }
 
-      #arrow[is-open] {
+      #arrow-icon[is-open] {
         transform: initial;
       }
 
@@ -82,7 +85,9 @@
         <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">