怎样用JS获取弹出层中dropdownlist的selectedvalue值
<div class=" Dialog" id="AddOrEditRoleDialog" style="top: 80px; width: 480px; display: none;
  text-align: center; margin: aotu;">
  <div class=" DialogHeader" onmousedown="javascript:Drag(this,$get('AddOrEditRoleDialog'),event)">
  <div class="DialogHeaderLeft">
  <div class="DialogExpandImage">
  <img src="../Resourse/Theme/images/add.gif" /></div>
  </div>
  <div id="PerAddMenu" class="DialogHeaderCenter">
  添加产品分类</div>
  <div id="PerEditRole" class="DialogHeaderCenter" style="display: none;">
  编辑产品名称</div>
  </div>
  <div class=" DialogContent" style="padding-top: 15px; padding-bottom: 15px;">
  <asp:UpdatePanel ID="UpdatePanelNewPerAssetReg" runat="server" UpdateMode="Conditional"
  ChildrenAsTriggers="false">
  <ContentTemplate>
  <table style="width: 90%;" class="styleTable">
  <tr>
  <td class="FormItemText">
  品牌名称：
  </td>
  <td class="FromItemLeft">
  <asp:DropDownList ID="DDL_CompanyName" runat="server" Width="155px">
  </asp:DropDownList>
  </td>
  <td class="FormItemText">
   
  </td>
  </tr>
  <tr>
  <td class="FormItemText">
  分类名称：
  </td>
  <td class="FromItemLeft">
  <asp:TextBox ID="TB_ProType" runat="server"></asp:TextBox>
  </td>
  <td class="FormItemText">
   
  </td>
  </tr>
   
  <tr>
  <td colspan="2" align="center">
  <asp:Button ID="SavePerRoletRegButton" AccessKey="F" onmouseover="javascript:this.className='ButtonOver'"
  onmouseout="javascript:this.className='Button'" runat="server" CssClass="Button"
  Text=" 添加(F) " OnClientClick="javascript:return checkSubmit();" OnClick="SavePerRoletRegButton_Click"
  UseSubmitBehavior="True" />&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;
  <input class=" Button" accesskey="C" onmouseover="javascript:t