Aura: notify DropHelper when target view is removed.

ui/aura_shell/shadow.h

Index: ui/aura_shell/shadow.h
diff --git a/ui/aura_shell/shadow.h b/ui/aura_shell/shadow.h
index 0fea19a47c2588164ebde8e5da3faa164206efb1..ba7328510f3da06a7013b7cede975553ae3aeea8 100644
--- a/ui/aura_shell/shadow.h
+++ b/ui/aura_shell/shadow.h
@@ -8,6 +8,7 @@
 
 #include "base/basictypes.h"
 #include "base/memory/scoped_ptr.h"
+#include "ui/aura_shell/aura_shell_export.h"
 #include "ui/gfx/rect.h"
 
 namespace ui {
@@ -20,7 +21,7 @@ namespace internal {
 class ImageGrid;
 
 // Simple class that draws a drop shadow around content at given bounds.
-class Shadow {
+class AURA_SHELL_EXPORT Shadow {
  public:
   Shadow();
   ~Shadow();