Split Screen mode implementation

athena/wm/window_stack_provider.h

Index: athena/wm/window_stack_provider.h
diff --git a/athena/wm/window_stack_provider.h b/athena/wm/window_stack_provider.h
new file mode 100644
index 0000000000000000000000000000000000000000..5815e8fa3a2ef7d8ed0275df508a3eea9c2754e7
--- /dev/null
+++ b/athena/wm/window_stack_provider.h
@@ -0,0 +1,21 @@
+#ifndef ATHENA_WM_WINDOW_STACK_PROVIDER_H_
+#define ATHENA_WM_WINDOW_STACK_PROVIDER_H_
+
+namespace aura {
+class Window;
+}
+
+namespace athena {
+
+// Interface for a stack of aura::Window objects.
+class WindowStackProvider {
+ public:
+  virtual ~WindowStackProvider() {}
+  virtual void MoveWindowToFront(aura::Window* window) = 0;
+
+  virtual const aura::Window::Windows& GetWindowStack() = 0;
+};
+
+}  // namespace athena
+
+#endif  // ATHENA_WM_WINDOW_STACK_PROVIDER_H_