Reenable device-emulation-scale-only test on Mac.

BUG=356743
TBR=pfeldman@chromium.org

Review URL: https://codereview.chromium.org/230623002

git-svn-id: svn://svn.chromium.org/blink/trunk@171171 bbb929c8-8fbe-4397-9dbb-9b2b20218538