Set DragDropTracker's capture window's size to zero, otherwise, it gets

ash/drag_drop/drag_drop_tracker.cc

Index: ash/drag_drop/drag_drop_tracker.cc
diff --git a/ash/drag_drop/drag_drop_tracker.cc b/ash/drag_drop/drag_drop_tracker.cc
index 0de110ae9d5f28c6a81ad19c5adfcb40b5d7c4aa..7fa05370c178fef9a0a2ea035bce70080054c405 100644
--- a/ash/drag_drop/drag_drop_tracker.cc
+++ b/ash/drag_drop/drag_drop_tracker.cc
@@ -19,7 +19,8 @@ aura::Window* CreateCaptureWindow(aura::RootWindow* root_window) {
   window->SetType(aura::client::WINDOW_TYPE_NORMAL);
   window->Init(ui::LAYER_NOT_DRAWN);
   window->SetParent(NULL);
-  window->SetBoundsInScreen(root_window->GetBoundsInScreen(),
+  window->SetBoundsInScreen(gfx::Rect(root_window->GetBoundsInScreen().origin(),
+                                      gfx::Size()),
                             gfx::Screen::GetDisplayNearestWindow(root_window));
   window->Show();
   return window;