Introduce UndoStack class

This patch introduces UndoStack class moved from EditorClientImpl class to make Blink API layer thin for ease of maintenance.

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

Review URL: https://codereview.chromium.org/74753003

git-svn-id: svn://svn.chromium.org/blink/trunk@162433 bbb929c8-8fbe-4397-9dbb-9b2b20218538