Use const Node reference for member functions in DocumentMarkerController

This patch changes Node pointer parameter to const Node reference in member functions in DocumentController because these parameters are never null for ease of maintenance.

This patch is a part of using const Node reference for Document::did{Insert,Remove,Merge,Split}Text(), http://crrev.com/24023002.

BUG=n/a
TEST=n/a; no behavior changes

Review URL: https://codereview.chromium.org/80273002

git-svn-id: svn://svn.chromium.org/blink/trunk@162460 bbb929c8-8fbe-4397-9dbb-9b2b20218538