Set minimum SDK level to 10.7 for Mac and iOS.

This is needed since r7174 introduced a dependency
on AVFoundation, which is not present in the 10.6 SDK which is
still the default for Chromium.

BUG=
TESTED=Passing compile on trybots.
R=henrika@webrtc.org

Review URL: https://webrtc-codereview.appspot.com/22659004

git-svn-id: http://webrtc.googlecode.com/svn/trunk@7175 4adac7df-926f-26a2-2b94-8c16560cd09d