In its latest effort to boost advertising by small businesses, Google said its YouTube unit had acquired Directr, a mobile-video app used by small businesses to create marketing videos. No purchase price was disclosed. Directr offers a mobile app for Apple’s iOS platform that makes it easy for small businesses to shoot, edit and upload short videos to the Internet. Google said Directr’s employees will join YouTube’s video-ads team. Directr now charges for most versions of its app, but Google said all versions will be free once the start-up joins YouTube. The Directr deal is the latest example of Google seeking to add tools for small businesses to encourage them to run more advertising across its sites. The company’s Google My Business suite of tools is geared toward helping small businesses appear in Google search and Google Maps.