Hide RenderBox::isBox() / RenderText::isText()

Hide RenderBox::isBox() / RenderText::isText() to catch useless type checks
at compile time.

R=pdr@chromium.org

Review URL: https://codereview.chromium.org/425503002

git-svn-id: svn://svn.chromium.org/blink/trunk@179037 bbb929c8-8fbe-4397-9dbb-9b2b20218538