Move the concept of Activation to the Shell.

ui/aura_shell/workspace/workspace.cc

Index: ui/aura_shell/workspace/workspace.cc
===================================================================
--- ui/aura_shell/workspace/workspace.cc	(revision 114022)
+++ ui/aura_shell/workspace/workspace.cc	(working copy)
@@ -11,6 +11,7 @@
 #include "ui/aura/root_window.h"
 #include "ui/aura/window.h"
 #include "ui/aura_shell/property_util.h"
+#include "ui/aura_shell/window_util.h"
 #include "ui/aura_shell/workspace/workspace_manager.h"
 #include "ui/base/ui_base_types.h"
 #include "ui/gfx/compositor/layer.h"
@@ -95,7 +96,7 @@
 
 aura::Window* Workspace::FindRotateWindowForLocation(
     const gfx::Point& position) {
-  aura::Window* active = aura::RootWindow::GetInstance()->active_window();
+  aura::Window* active = aura_shell::GetActiveWindow();
   if (GetTotalWindowsWidth() < bounds_.width()) {
     // If all windows fit to the width of the workspace, it returns the
     // window which contains |position|'s x coordinate.