webrtc/base: FileModifyTime -> OlderThan as that's what it was ever used as. Needed for cl/70828325.

BUG=N/A
R=andrew@webrtc.org

Review URL: https://webrtc-codereview.appspot.com/21939004

git-svn-id: http://webrtc.googlecode.com/svn/trunk@6787 4adac7df-926f-26a2-2b94-8c16560cd09d