aura: No shadow for transparent window.

ui/aura/window.cc

Index: ui/aura/window.cc
diff --git a/ui/aura/window.cc b/ui/aura/window.cc
index f5f64626d6ab507022150dca7094d221ae149c59..59914298dc31b3a2446e4fd12bada78fa8d83fc3 100644
--- a/ui/aura/window.cc
+++ b/ui/aura/window.cc
@@ -41,6 +41,7 @@ Window::Window(WindowDelegate* delegate)
       parent_(NULL),
       transient_parent_(NULL),
       id_(-1),
+      transparent_(false),
       user_data_(NULL),
       stops_event_propagation_(false),
       ignore_events_(false) {
@@ -100,6 +101,7 @@ void Window::Init(ui::Layer::LayerType layer_type) {
   layer_->SetVisible(false);
   layer_->set_delegate(this);
   UpdateLayerName(name_);
+  layer_->SetFillsBoundsOpaquely(!transparent_);
 
   RootWindow::GetInstance()->WindowInitialized(this);
 }
@@ -117,6 +119,12 @@ void Window::SetName(const std::string& name) {
     UpdateLayerName(name_);
 }
 
+void Window::SetTransparent(bool transparent) {
+  // Cannot change transparent flag after the window is initialized.
+  DCHECK(!layer());
+  transparent_ = transparent;
+}
+
 ui::Layer* Window::AcquireLayer() {
   return layer_.release();
 }