Chrome Extension - Get page source when fully loaded (client side content too) - google-chrome-extension

Question

I'm developing a Chrome Extension where I need to retrieve the source code of a newly create tab.
Retrieving the source code is not a problem as such, my script below does that perfectly but what I'm looking for is to get the source code once the page has been completely loaded included dynamic content that has been inserted after page load (like with Ajax for example).
To get the source code, I'm using
chrome.tabs.executeScript(tabId,
{
file: 'test.js',
}, function(results)
{
// DO SOMETHING HERE
}
test.js contains the following:
function DOMtoString(document_root) {
var html = '',
node = document_root.firstChild;
while (node) {
switch (node.nodeType) {
case Node.ELEMENT_NODE:
html += node.outerHTML;
break;
case Node.TEXT_NODE:
html += node.nodeValue;
break;
case Node.CDATA_SECTION_NODE:
html += '<![CDATA[' + node.nodeValue + ']]>';
break;
case Node.COMMENT_NODE:
html += '<!--' + node.nodeValue + '-->';
break;
case Node.DOCUMENT_TYPE_NODE:
// (X)HTML documents are identified by public identifiers
html += "<!DOCTYPE " + node.name + (node.publicId ? ' PUBLIC "' + node.publicId + '"' : '') + (!node.publicId && node.systemId ? ' SYSTEM' : '') + (node.systemId ? ' "' + node.systemId + '"' : '') + '>\n';
break;
}
node = node.nextSibling;
}
return html;
}
chrome.runtime.sendMessage({
action: "getSource",
source: DOMtoString(document),
metadata: shop_metadata
});
What I would like to do is to retrieve the source code only when a class has been detected and if it failed after a couple of seconds, the script should skip the process. The class I'm looking for is available in the "shop_metadata" variable.
Is there a way to make that happen with SendMessage like in my script?
I have tried to do this:
function test (document)
{
var checkExist = setInterval(function()
{
if ($('.product-title').length)
{
console.log("Exists!");
source = DOMtoString(document);
console.log(source);
return source;
clearInterval(checkExist);
}
}, 5000);
}
and then modify my message:
chrome.runtime.sendMessage({
action: "getSource",
source:test(document),
metadata: shop_metadata
});
If I executed this in the console, I get the source code with the dynamic content but when I checked the content of "source" in the extension, it returns "undefined"
Thanks
Laurent