aura: No shadow for transparent window.

ui/aura/window.h

Index: ui/aura/window.h
diff --git a/ui/aura/window.h b/ui/aura/window.h
index 9fc775ead32579668b6072b95a6099c48fdfc800..9d3a2fdf97b3785b581390810e6d8414ee5776ff 100644
--- a/ui/aura/window.h
+++ b/ui/aura/window.h
@@ -70,6 +70,9 @@ class AURA_EXPORT Window : public ui::LayerDelegate {
   const string16 title() const { return title_; }
   void set_title(const string16& title) { title_ = title; }
 
+  bool transparent() const { return transparent_; }
+  void SetTransparent(bool transparent);
+
   ui::Layer* layer() { return layer_.get(); }
   const ui::Layer* layer() const { return layer_.get(); }
 
@@ -341,6 +344,9 @@ class AURA_EXPORT Window : public ui::LayerDelegate {
 
   string16 title_;
 
+  // Whether layer is initialized as non-opaque.
+  bool transparent_;
+
   scoped_ptr<EventFilter> event_filter_;
   scoped_ptr<LayoutManager> layout_manager_;