Get rid of unused method |EditingContext.prototype.cloneNode|.
Get rid of unused getter |EditingContext.prototype.editor|.
Make |EditingContext| independent from |Editor|.
Add missing JS externs for |EditingContext.prototype.shouldUseCSS| getter.
Simply JS externs for |Editor|.
[JS Externs] Introduce |Document.prototype.contains(Node):boolean|.
Note: Closure compiler has a wrong JS externs |Element.prototype.contains(Element):boolean| instead of |Node.prototype.contains()|.
Split |EditingContext| into |EditingContext| and |UndoUnit| for ease of reviewing.
Fix value for |constructor| property for object prototype.