Use active window if on current workspace for fullscreen mode.

ash/root_window_controller.h

Index: ash/root_window_controller.h
diff --git a/ash/root_window_controller.h b/ash/root_window_controller.h
index aa1ec2bc5a0b27692a69123bb258557a64f8a465..5e017aca2fdfbc9df4c9efa21afceec59432061a 100644
--- a/ash/root_window_controller.h
+++ b/ash/root_window_controller.h
@@ -110,7 +110,12 @@ class ASH_EXPORT RootWindowController : public ShellObserver {
   virtual ~RootWindowController();
 
   aura::Window* root_window() { return dispatcher()->window(); }
+  const aura::Window* root_window() const { return dispatcher()->window(); }
+
   aura::WindowEventDispatcher* dispatcher() { return root_window_.get(); }
+  const aura::WindowEventDispatcher* dispatcher() const {
+    return root_window_.get();
+  }
 
   RootWindowLayoutManager* root_window_layout() { return root_window_layout_; }