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 db1f54399664886a0bbc753be79565fe9b04cfbc..340d54c299f05cdaee47a0d215b6cd26ec1de379 100644
--- a/apps/app_window.cc
+++ b/apps/app_window.cc
@@ -245,7 +245,8 @@ AppWindow::AppWindow(BrowserContext* context,
       has_been_shown_(false),
       can_send_events_(false),
       is_hidden_(false),
-      cached_always_on_top_(false) {
+      cached_always_on_top_(false),
+      requested_transparent_background_(false) {
   extensions::ExtensionsBrowserClient* client =
       extensions::ExtensionsBrowserClient::Get();
   CHECK(!client->IsGuestSession(context) || context->IsOffTheRecord())
@@ -285,6 +286,8 @@ 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));
 
   popup_manager_.reset(
@@ -756,6 +759,9 @@ 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