ash: Add RootWindowHostFactory class.

ash/shell_delegate.h

Index: ash/shell_delegate.h
diff --git a/ash/shell_delegate.h b/ash/shell_delegate.h
index a7eb83872619d35e0552ef59b52707c29420f0bc..c056d86db7886a64ab8e5eada86f284ef348b5a8 100644
--- a/ash/shell_delegate.h
+++ b/ash/shell_delegate.h
@@ -20,6 +20,7 @@ class AppListViewDelegate;
 
 namespace aura {
 class RootWindow;
+class RootWindowDelegate;
 class Window;
 namespace client {
 class StackingClient;
@@ -212,6 +213,10 @@ class ASH_EXPORT ShellDelegate {
   // True if the user's preferences have the Search key acting as a Function key
   // modifier for accessing extended keyboard shortcuts.
   virtual bool IsSearchKeyActingAsFunctionKey() const = 0;
+
+  // Creates a root window delegate. Shell takes ownership of the returned
+  // value.
+  virtual aura::RootWindowDelegate* CreateRootWindowDelegate() = 0;
 };
 
 }  // namespace ash