WebDriver cannot be used todo browser specific actions, it can only be used to drive the web-page. Although sending the keys CTRL-TAB might switch tabs WebDriver will still think you are in the previous window.
Switching Tabs as you want is thus actually something different. Each tab is actually a window organised in a tab like look-and-feel. Thus you have to use
driver.switchTo().window('windowName'); to switch "tabs" and to find the window names use
driver.getAllWindowHandles() which returns a list of windows.
Would look something like this:
This thread also shows the difference between WebDriverJS and WebDriver.IO including examples how-to use the keyDown.