[MediaGalleries] Clean up dialog notifications from/to controller.

Fixes a bug with handling multiple folder adds in the dialog.

Review URL: https://codereview.chromium.org/30333002

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@230510 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 230510