linux aura: Fix notification popups.

Make sure notification popups do not get tiled in tiling window managers.

BUG=318810
R=erg@chromium.org

Review URL: https://codereview.chromium.org/71793002

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@234885 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 234885