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..d619540ada0af325d4f79ef331b94a44f4b7c44f 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())
@@ -284,6 +285,8 @@ 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 +745,10 @@ void AppWindow::WindowEventsReady() {
   SendOnWindowShownIfShown();
 }
 
+bool AppWindow::requested_transparent_background() const {
+  return requested_transparent_background_;

[tapted, 2014/07/16 03:01:01]

nit: inline this in the header? (ignoring names, this looks more correct for the
follow-up in https://codereview.chromium.org/190533006/ too).

[jackhou1, 2014/07/16 05:54:04]

Done.

+}
+
 void AppWindow::GetSerializedState(base::DictionaryValue* properties) const {
   DCHECK(properties);
 
@@ -751,6 +758,9 @@ 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