Don't pretend that text-decoration is a shorthand in computed style.

Even when the appropriate runtime flag isn't enabled, we still return
the shorthand version of the computed style for 'text-decoration'. For
example, for 'text-decoration: underline', we return the computed style
'underline solid rgb(0, 0, 0)', which is correct only when the runtime
flag is on.

BUG=367091,342126

Review URL: https://codereview.chromium.org/282303007

git-svn-id: svn://svn.chromium.org/blink/trunk@174269 bbb929c8-8fbe-4397-9dbb-9b2b20218538