Make Element::synchronizeAllAttributes() private

Make Element::synchronizeAllAttributes() private instead of public. Now that
Element::attributes() calls synchronizeAllAttributes() internally, there is
no reason for external code to call it.

R=adamk@chromium.org

Review URL: https://codereview.chromium.org/448063004

git-svn-id: svn://svn.chromium.org/blink/trunk@179752 bbb929c8-8fbe-4397-9dbb-9b2b20218538