Win8: Ensure correct desktop for various notification center widgets.

Currently all widgets in the regular views hierarchy are homed on the
"Active Desktop" which may be the Metro/Ash desktop.  This creates
duplicate views of the notifications since we also have a separate
infrastructure for showing notifications in Ash.

BUG=322690,321605

Review URL: https://codereview.chromium.org/84083003

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@237401 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 237401