[UndoManager] What should a "native" automatic transaction expose?

Hi,

In section 3.3 [1], we mention that the user editing actions and drag and
drop need to be implemented as automatic DOM transactions. But it seems odd
to expose executeAutomatic function in this case especially for drag & drop.

I propose to set executeAutomatic null for those "native" automatic
transactions but still expose label (set to whatever the Web browsers use
in the edit menu).

[1]
http://dvcs.w3.org/hg/undomanager/raw-file/tip/undomanager.html#automatic-dom-transactions

Best,
Ryosuke Niwa
Software Engineer
Google Inc.