Move style building for grid-auto-flow to StyleBuilderConverter.

There doesn't appear to be a reason for this property to have
custom behavior. It's better to use a converter and call that
from the generated function.

Review URL: https://codereview.chromium.org/642303002

git-svn-id: svn://svn.chromium.org/blink/trunk@183595 bbb929c8-8fbe-4397-9dbb-9b2b20218538