Follow up recent changes on DesktopNativeWidgetAura for Ozone

CL 31043006 moved a lot of code to DesktopNativeWidgetAura and changed
DesktopRootWindowHost but forgot to update the Ozone backend. This CL fix such
behaviour.

BUG=none

Review URL: https://codereview.chromium.org/44113004

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@231340 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 231340