Fix shell_aura.cc with RootWindow refactor

To get aura::Window we need to call root_window_->window().

NOTRY=true

Review URL: https://codereview.chromium.org/72013002

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@235254 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 235254