how to get current tabId from background page

getSelected has been deprecated. The new way to do it is:

chrome.tabs.query(
  {currentWindow: true, active : true},
  function(tabArray){...}
)

If you want to perform some callback on the active tab, you can wrap the above as so:

function doInCurrentTab(tabCallback) {
    chrome.tabs.query(
        { currentWindow: true, active: true },
        function (tabArray) { tabCallback(tabArray[0]); }
    );
}

For example

var activeTabId;
doInCurrentTab( function(tab){ activeTabId = tab.id } );