Add whitespace.txt file.

This is useful as a recommended way to trigger now builds
with a noop change.
I believe it's going to be used more frequently as we're closing
in on the Git switch, to test committing and pushing.

TBR=phoglund@webrtc.org
BUG=

Review URL: https://webrtc-codereview.appspot.com/32559004

git-svn-id: http://webrtc.googlecode.com/svn/trunk@7764 4adac7df-926f-26a2-2b94-8c16560cd09d