Don't use RenderStyle::clone unless there is actually a RenderObject to attach the cloned style to.

Non-renderer-based elements such as optGroup do not have renderers.
BUG=348330
R=esprehn@chromium.org

Review URL: https://codereview.chromium.org/185293011

git-svn-id: svn://svn.chromium.org/blink/trunk@168394 bbb929c8-8fbe-4397-9dbb-9b2b20218538