Mark external-paintserver-reference.svg for rebaselining on Windows.

Apparently the recent rebaselining only did the right thing for XP
and not for Win7. Let's try that again.

TBR=fs@chromium.org
NOTRY=true
BUG=416580

Review URL: https://codereview.chromium.org/601143003

git-svn-id: svn://svn.chromium.org/blink/trunk@182671 bbb929c8-8fbe-4397-9dbb-9b2b20218538