Bookmarks: fix splitter right mousedown issue

ui/webui/resources/js/cr/ui/splitter.js

Index: ui/webui/resources/js/cr/ui/splitter.js
diff --git a/ui/webui/resources/js/cr/ui/splitter.js b/ui/webui/resources/js/cr/ui/splitter.js
index 2c1799e4f2216ce4ef0812c2d6f2b5e7e9929296..aacfacee304d46292056389c6814614bac8267eb 100644
--- a/ui/webui/resources/js/cr/ui/splitter.js
+++ b/ui/webui/resources/js/cr/ui/splitter.js
@@ -135,6 +135,8 @@ cr.define('cr.ui', function() {
      */
     handleMouseDown_: function(e) {
       e = /** @type {!MouseEvent} */(e);
+      if (e.button)
+        return;
       this.startDrag(e.clientX, false);
       // Default action is to start selection and to move focus.
       e.preventDefault();