Revert 233787 "Eliminate Shell::RootWindowList in favor of aura:..."

> Eliminate Shell::RootWindowList in favor of aura::Window::Windows.
> 
> This is necessary as part of my plan to rework RootWindow, specifically making it not be a subclass of Window. (It will eventually be renamed WindowEventDispatcher). Code that expected to get the RootWindow interface from this type can still call GetDispatcher() on members of the returned vector of Windows.
> 
> R=oshima@chromium.org
> http://crbug.com/308843
> 
> Review URL: https://codereview.chromium.org/64933002

TBR=ben@chromium.org

Review URL: https://codereview.chromium.org/59153004

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@233813 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 233813