Add app.window.alphaEnabled() and onAlphaEnabledChanged.

apps/app_window.cc

Index: apps/app_window.cc
diff --git a/apps/app_window.cc b/apps/app_window.cc
index e2d837d4e3847cda39e2cf9954fccc150f5931b4..f475bb093cb8854c68428970d5b84797edcc17a3 100644
--- a/apps/app_window.cc
+++ b/apps/app_window.cc
@@ -284,6 +284,8 @@ void AppWindow::Init(const GURL& url,
   if (new_params.state == ui::SHOW_STATE_FULLSCREEN)
     new_params.always_on_top = false;
 
+  requested_transparent_background_ = new_params.transparent_background;
+
   native_app_window_.reset(delegate_->CreateNativeAppWindow(this, new_params));
 
   // Prevent the browser process from shutting down while this window exists.
@@ -742,6 +744,10 @@ void AppWindow::WindowEventsReady() {
   SendOnWindowShownIfShown();
 }
 
+bool AppWindow::RequestedTransparentBackground() const {
+  return requested_transparent_background_;
+}
+
 void AppWindow::GetSerializedState(base::DictionaryValue* properties) const {
   DCHECK(properties);
 
@@ -751,6 +757,9 @@ void AppWindow::GetSerializedState(base::DictionaryValue* properties) const {
   properties->SetBoolean("maximized", native_app_window_->IsMaximized());
   properties->SetBoolean("alwaysOnTop", IsAlwaysOnTop());
   properties->SetBoolean("hasFrameColor", native_app_window_->HasFrameColor());
+  properties->SetBoolean("alphaEnabled",
+                         requested_transparent_background_ &&
+                             native_app_window_->CanHaveAlphaEnabled());
 
   // These properties are undocumented and are to enable testing. Alpha is
   // removed to