Don't try to rebaseline from debug bots.

We only ever rebaseline from release bots. If a debug bot is getting
different results from the release bot, then a rebaseline will not fix
it.

Instead of throwing a Python stack, print that we're skipping the debug
builders.

BUG=310236
NOTRY=true

Review URL: https://codereview.chromium.org/34713005

git-svn-id: svn://svn.chromium.org/blink/trunk@160261 bbb929c8-8fbe-4397-9dbb-9b2b20218538