Actually clear diff viewer after closing save dialog and changing document

The window refactor removed the 'teardown' event from windows,
but ViewPageTarget was still listening to this event on the save dialog.
Instead, use the opening promise to be notified when the save dialog
closes and call onSaveDialogClose() at that time.

Change-Id: I31a9a359eb14a56b214a1240db08632bda6bc8b9
(cherry picked from commit a6d94256d8504fb524fcdb646b333c2d0ca81321)