border-radius cssText always outputs to longhand properties

Prefer to keep the border-radius for cssText output if it is
explictly used.

Matches FireFox behavior.

BUG=407012

Review URL: https://codereview.chromium.org/547043002

git-svn-id: svn://svn.chromium.org/blink/trunk@181942 bbb929c8-8fbe-4397-9dbb-9b2b20218538