webrtc::Scaler: Preserve aspect ratio

BUG=3936
R=glaznev@webrtc.org, stefan@webrtc.org

Review URL: https://webrtc-codereview.appspot.com/28969004

git-svn-id: http://webrtc.googlecode.com/svn/trunk@7679 4adac7df-926f-26a2-2b94-8c16560cd09d