New Sync datatype for Synced Notifications App Info

This is the first checkin for creating a new sync datatype for the synced notifications app info.  The new datatype will be used for synced notification metadata such as the name and icon of the service sending the synced notification.

BUG=280266

Review URL: https://codereview.chromium.org/143973006

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@247551 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 247551
Clean up BrowserWindowController by using the ObjCCast helper function.

BUG=none
TEST=no change

Review URL: https://codereview.chromium.org/144343005

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@247553 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 247553