Stack at top functionality for Aura-based windows

Implementation of previously empty DesktopNativeWidgetAura::StackAtTop()
method.

BUG=

Review URL: https://codereview.chromium.org/32393002

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@238978 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 238978