Desktop Aura: Allow tab drags out of window.

ui/base/event.h

Index: ui/base/event.h
diff --git a/ui/base/event.h b/ui/base/event.h
index ef53a19a6db31c6acf27b7842eeed96913740f55..04f566a307c7e7daed2f3be36eacd43c37c716bf 100644
--- a/ui/base/event.h
+++ b/ui/base/event.h
@@ -145,6 +145,13 @@ class UI_EXPORT LocatedEvent : public Event {
   gfx::Point location() const { return location_; }
   gfx::Point root_location() const { return root_location_; }
 
+  bool valid_system_location() const {
+    return system_location_ != gfx::Point(-1, -1);
+  }
+  void set_system_location(const gfx::Point& loc) { system_location_ = loc; }
+  gfx::Point system_location() const { return system_location_; }
+
+
   // Applies |root_transform| to the event.
   // This is applied to both |location_| and |root_location_|.
   virtual void UpdateForRootTransform(const Transform& root_transform);
@@ -159,7 +166,10 @@ class UI_EXPORT LocatedEvent : public Event {
   LocatedEvent(const LocatedEvent& model, T* source, T* target)
       : Event(model),
         location_(model.location_),
-        root_location_(model.root_location_) {
+        root_location_(model.root_location_),
+        system_location_(model.system_location_) {
+    // TODO(erg): May need to create system_location_ by converting location to
+    // system coordinates here.
     if (target && target != source)
       T::ConvertPointToTarget(source, target, &location_);
   }
@@ -175,6 +185,8 @@ class UI_EXPORT LocatedEvent : public Event {
   gfx::Point location_;
 
   gfx::Point root_location_;
+
+  gfx::Point system_location_;
 };
 
 class UI_EXPORT MouseEvent : public LocatedEvent {