Removes specifying transient at the aura level for top levels

This causes problems as it means the aura::Window may be deleted out
from under the DesktopNativeWidgetAura, which causes all sorts of woe.

This code is no longer needed since DesktopWindowTreeHostX11 now deals
with owned windows.

BUG=344265
TEST=none
R=erg@chromium.org

Review URL: https://codereview.chromium.org/186483007

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@255377 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 255377