Select the pointed item when drag operation starts.

What this CL do:
 If there is an item at the drag's starting position and
the item is not selected, select the item.

Why this handling is needed:
 If the drag operation is initiated by mouse, the
'mousedown' event eventually selects the pointed item,
but there is no such chance when the drag operation is 
initiated by touch.

BUG=373839
TEST=manually tested

Review URL: https://codereview.chromium.org/288883004

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@271535 0039d316-1c4b-4281-b951-d872f2087c98

ui/webui/resources/js/cr/ui/list.js

@@ -314,6 +314,7 @@ cr.define('cr.ui', function() {
 
       this.addEventListener('dblclick', this.handleDoubleClick_);
       this.addEventListener('mousedown', handleMouseDown);
+      this.addEventListener('dragstart', handleDragStart, true);
       this.addEventListener('mouseup', this.handlePointerDownUp_);
       this.addEventListener('keydown', this.handleKeyDown);
       this.addEventListener('focus', this.handleElementFocus_, true);
@@ -1302,6 +1303,24 @@ cr.define('cr.ui', function() {
     }
   }
 
+  /**
+   * Dragstart event handler.
+   * If there is an item at starting position of drag operation and the item
+   * is not selected, select it.
+   * @this {List}
+   * @param {MouseEvent} e The event object for 'dragstart'.
+   */
+  function handleDragStart(e) {
+    var element = e.target.ownerDocument.elementFromPoint(e.clientX, e.clientY);
+    var target = this.getListItemAncestor(element);
+    var index = this.getIndexOfListItem(target);
+    if (index != -1) {
+      var isAlreadySelected = this.selectionModel_.getIndexSelected(index);
+      if (!isAlreadySelected)
+        this.selectionModel_.selectedIndex = index;
+    }
+  }
+
   /**
    * Check if |start| or its ancestor under |root| is focusable.
    * This is a helper for handleMouseDown.