Fix missing HTML Tooltip ContentMode for Tree items (#10677)

server/src/main/java/com/vaadin/ui/Tree.java

         treeGrid.setDescriptionGenerator(descriptionGenerator);
     }
 
+    /**
+     * Sets the description generator that is used for generating HTML tooltip
+     * descriptions for items.
+     *
+     * @param descriptionGenerator
+     *            the item description generator to set, or <code>null</code> to
+     *            remove a previously set generator
+     * @param contentMode
+     *            how client should interpret textual values
+     *
+     * @since
+     */
+    public void setItemDescriptionGenerator(
+            DescriptionGenerator<T> descriptionGenerator,
+            ContentMode contentMode) {
+        treeGrid.setDescriptionGenerator(descriptionGenerator, contentMode);
+    }
+
     /**
      * Gets the item caption generator.
      *