Drop editing's createFullMarkup() function

Drop editing's createFullMarkup() function and inline it into the caller
instead. There is only one caller and that caller passes a Document to the
function so the function ends up doing a simple call to
createMarkup(node).

R=adamk@chromium.org, leviw@chromium.org

Review URL: https://codereview.chromium.org/430823002

git-svn-id: svn://svn.chromium.org/blink/trunk@179244 bbb929c8-8fbe-4397-9dbb-9b2b20218538