Makes destruction of DesktopNativeWidgetAura saner

Prior to this patch DesktopNativeWidgetAura destruction keyed off
destruction of the window_ (not the rootwindow). This meant that by
the time we destroyed the Widget the RootWindow was still live. This
meant any children of the rootwindow, such as child widgets, were
alive as well. The general expectation is that by the time a Widget is
destroyed and child Widgets have also been destroyed.

This patch changes destruction so that we key off the
AcceleratedWidget going away. When that happens we destroy the
RootWindow. By doing this we ensure child Widgets have also been
destroyed.

BUG=288953
TEST=covered by test
R=ben@chromium.org

Review URL: https://codereview.chromium.org/24515004

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@225755 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 225755