[MD Bookmarks] Make shift-selection behave well with no anchor.

chrome/browser/resources/md_bookmarks/list.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-list',
 
   behaviors: [
     bookmarks.StoreClient,
   ],
@@ skipping 112 matching lines @@
   /**
    * @param {KeyboardEvent} e
    * @private
    */
   onItemKeydown_: function(e) {
     var handled = true;
     var list = this.$.bookmarksCard;
     var focusMoved = false;
     var focusedIndex =
         this.getIndexForItemElement_(/** @type {HTMLElement} */ (e.target));
+    var oldFocusedIndex = focusedIndex;
     if (e.key == 'ArrowUp') {
       focusedIndex--;
       focusMoved = true;
     } else if (e.key == 'ArrowDown') {
       focusedIndex++;
       focusMoved = true;
       e.preventDefault();
     } else if (e.key == 'Home') {
       focusedIndex = 0;
       focusMoved = true;
@@ skipping 16 matching lines @@
     }
 
     if (focusMoved) {
       focusedIndex = Math.min(list.items.length - 1, Math.max(0, focusedIndex));
       list.focusItem(focusedIndex);
 
       if (e.ctrlKey && !e.shiftKey) {
         this.dispatch(
             bookmarks.actions.updateAnchor(this.displayedIds_[focusedIndex]));
       } else {
+        // If shift-selecting with no anchor, use the old focus index.
+        if (e.shiftKey && this.getState().selection.anchor == null) {
+          this.dispatch(bookmarks.actions.updateAnchor(
+              this.displayedIds_[oldFocusedIndex]));
+        }
+
         // If the focus moved from something other than a Ctrl + move event,
         // update the selection.
         var config = {
           clear: !e.ctrlKey,
           range: e.shiftKey,
           toggle: false,
         };
 
         this.dispatch(bookmarks.actions.selectItem(
             this.displayedIds_[focusedIndex], this.getState(), config));
       }
     }
 
     if (handled)
       e.stopPropagation();
   },
 });