WebUI: Prevent text selection on action menu entries.

ui/webui/resources/cr_elements/cr_action_menu/cr_action_menu.html

 <link rel="import" href="chrome://resources/html/util.html">
 <link rel="import" href="chrome://resources/html/polymer.html">
 <link rel="import" href="chrome://resources/cr_elements/shared_vars_css.html">
 
 <dom-module id="cr-action-menu">
   <template>
     <style>
       :host {
         background-color: white;
         border: none;
@@ skipping 10 matching lines @@
 
       :host ::content .dropdown-item {
         background: none;
         border: none;
         box-sizing: border-box;
         color: inherit;
         font: inherit;
         min-height: 32px;
         padding: 0 24px;
         text-align: start;
+        user-select: none;
         width: 100%;
       }
 
       :host ::content .dropdown-item:not([hidden]) {
         align-items: center;
         display: flex;
       }
 
       :host ::content .dropdown-item[disabled] {
         opacity: 0.65;
@@ skipping 11 matching lines @@
       .item-wrapper {
         outline: none;
       }
     </style>
     <div class="item-wrapper" tabindex="-1">
       <content select=".dropdown-item,hr"></content>
     </div>
   </template>
   <script src="cr_action_menu.js"></script>
 </dom-module>