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

chrome/browser/resources/md_bookmarks/folder_node.js

 // Copyright 2016 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 Polymer({
   is: 'bookmarks-folder-node',
 
   behaviors: [
     bookmarks.StoreClient,
   ],
@@ skipping 67 matching lines @@
     return this.depth == 0;
   },
 
   /**
    * @private
    * @param {!Event} e
    */
   onKeydown_: function(e) {
     var direction = 0;
     var handled = true;
-    // TODO(calamity): Handle left/right arrow keys.
     if (e.key == 'ArrowUp') {
       direction = -1;
     } else if (e.key == 'ArrowDown') {
       direction = 1;
+    } else if (e.key == 'ArrowRight') {

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

These shortcuts need to flip in RTL.

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

Done.

+      // The right arrow opens a folder if closed and goes to the first child
+      // otherwise.
+      if (this.hasChildFolder_()) {
+        if (this.isClosed_) {
+          this.dispatch(
+              bookmarks.actions.changeFolderOpen(this.item_.id, true));
+        } else {
+          direction = 1;
+        }
+      }
+    } else if (e.key == 'ArrowLeft') {
+      // The left arrow closes a folder if open and goes to the parent
+      // otherwise.
+      if (this.hasChildFolder_() && !this.isClosed_) {
+        this.dispatch(bookmarks.actions.changeFolderOpen(this.item_.id, false));
+      } else {
+        var parentFolderNode = this.getParentFolderNode_();
+        if (parentFolderNode.itemId != ROOT_NODE_ID) {
+          parentFolderNode.selectFolder_();
+          parentFolderNode.getFocusTarget().focus();
+        }
+      }
     } else {
       handled = false;
     }
 
     if (direction)
       this.changeKeyboardSelection_(direction, this.root.activeElement);
 
     if (!handled)
       return;
 
@@ skipping 189 matching lines @@
   },
 
   /**
    * @private
    * @return {string}
    */
   getTabIndex_: function() {
     return this.isSelectedFolder_ ? '0' : '';
   },
 });