Add Mac to the flaky assertion failure in debugger-script-preprocessor.html

BUG=264934
TBR=pfeldman

Review URL: https://codereview.chromium.org/21103003

git-svn-id: svn://svn.chromium.org/blink/trunk@155089 bbb929c8-8fbe-4397-9dbb-9b2b20218538