Mark inspector/sources/debugger/script-formatter.html flaky

BUG=395137
TBR=pfeldman@chromium.org
NOTRY=true

Review URL: https://codereview.chromium.org/397243005

git-svn-id: svn://svn.chromium.org/blink/trunk@178453 bbb929c8-8fbe-4397-9dbb-9b2b20218538