roll_webrtc.py: Improve no-op roll message.

Currently there's no information printed when the
execution of the roll script is a no-op (e.g. when
the latest revision is already rolled).

BUG=

Review URL: https://codereview.chromium.org/1172323002

Cr-Commit-Position: refs/heads/master@{#333722}
1 file changed