First shot at implementing drag&drop for Aura

ui/aura/window.cc

Index: ui/aura/window.cc
diff --git a/ui/aura/window.cc b/ui/aura/window.cc
index d1caa0416e6eab3ea3e35464644793eef6d29341..5733813c45c1773bdc21b5acf49916bc0daf4da0 100644
--- a/ui/aura/window.cc
+++ b/ui/aura/window.cc
@@ -30,10 +30,15 @@ Window::Window(WindowDelegate* delegate)
       transient_parent_(NULL),
       id_(-1),
       user_data_(NULL),
-      stops_event_propagation_(false) {
+      stops_event_propagation_(false),
+      drag_drop_delegate_(NULL) {
 }
 
 Window::~Window() {
+  // We need to NULL the drag drop delegate to prevent race condition where a
+  // drag/drop event may be sent to a window while its being destroyed.
+  drag_drop_delegate_ = NULL;
+
   // Let the delegate know we're in the processing of destroying.
   if (delegate_)
     delegate_->OnWindowDestroying();