function screenshot(){
var a = '';
    chrome.extension.sendRequest({screenshot:"true"}, function(response) {
        if(response.url)var a = response.url;
    });
return a;
}

chrome.extension.onRequest.addListener(
  function(request, sender, sendResponse) {
    if(request.screenshot != ""){
        chrome.tabs.captureVisibleTab(sender.windowId,function(data){
        sendResponse({url: data});
        });
        }
});