Make Element::elementData() / ensureUniqueElementData() protected

Make Element::elementData() / ensureUniqueElementData() protected instead of
public as ElementData is an Element internal and outside classes should
probably not deal with it directly.

R=adamk@chromium.org

Review URL: https://codereview.chromium.org/451733002

git-svn-id: svn://svn.chromium.org/blink/trunk@179748 bbb929c8-8fbe-4397-9dbb-9b2b20218538