Cross Brower Issue with Iframe

Guys i am sitting for more than 2 days i couldnt able to get the solution for this,could you help me pls.......

I am having master page => MasterPage.master

under this one content page => Default.aspx

Another content page => AccountHelper.aspx

In default.aspx page i have one iframe html element, in that i am loading AccountHelper.aspx page.

 

In AccountHelper.aspx page i am having one hidden field like

 

Now my aim is to get the value of that hidden field from AccountHelper.aspx and display as an alert in Default.aspx page. 

 

I work around like this:

var parentDocumentFrame = document.getElementById('ifrETAOutId');
var contentElement = parentDocumentFrame.contentDocument || parentDocumentFrame.contentWindow.document; 

 contentElement.getElementbyId("hdnco").value;

 

Here now iam getting the value in IE but not in chrome.

since that hidden field is an server control it is rendering like following

In IE: _ct10_CntMasterPage_hdnco

In Chrome : ct100_ContentPlaceHolder1_hdnCo

 

i have also tried like this '<%= hdnco.ClientID %>' but still not working in chrome.