Add context to gfx::Screen calls in support of simultaneous desktop+ash

chrome/browser/ui/views/tabs/tab_drag_controller.h

Index: chrome/browser/ui/views/tabs/tab_drag_controller.h
diff --git a/chrome/browser/ui/views/tabs/tab_drag_controller.h b/chrome/browser/ui/views/tabs/tab_drag_controller.h
index 3a68162667625af20a0fea2407daacafbafcb415..d645b61d309a7acd9ca7e2494f9738c60a05bc98 100644
--- a/chrome/browser/ui/views/tabs/tab_drag_controller.h
+++ b/chrome/browser/ui/views/tabs/tab_drag_controller.h
@@ -21,6 +21,9 @@
 #include "ui/gfx/rect.h"
 #include "ui/views/widget/widget_observer.h"
 
+namespace gfx {
+class Screen;
+}
 namespace views {
 class View;
 }
@@ -471,6 +474,10 @@ class TabDragController : public content::WebContentsDelegate,
   // dragged Tab is detached.
   TabStrip* attached_tabstrip_;
 
+  // The screen that this drag is associated with. Cached, because other UI
+  // elements are NULLd at various points during the lifetime of this object.
+  gfx::Screen* screen_;
+
   // The visual representation of the dragged Tab.
   scoped_ptr<DraggedTabView> view_;