Remove undefined StyleResolverState::initElement() method

Remove StyleResolverState::initElement() method as it is not used nor
defined.

R=adamk

Review URL: https://codereview.chromium.org/49153005

git-svn-id: svn://svn.chromium.org/blink/trunk@160741 bbb929c8-8fbe-4397-9dbb-9b2b20218538