Add app.window.alphaEnabled() and onAlphaEnabledChanged.

apps/ui/views/native_app_window_views.cc

Index: apps/ui/views/native_app_window_views.cc
diff --git a/apps/ui/views/native_app_window_views.cc b/apps/ui/views/native_app_window_views.cc
index 62b941d9e58db4628f080934c6bf49b1660b76a8..36542204658950758864f6c4bded6d34d771630b 100644
--- a/apps/ui/views/native_app_window_views.cc
+++ b/apps/ui/views/native_app_window_views.cc
@@ -27,14 +27,12 @@ NativeAppWindowViews::NativeAppWindowViews()
       web_view_(NULL),
       widget_(NULL),
       frameless_(false),
-      transparent_background_(false),
       resizable_(false) {}
 
 void NativeAppWindowViews::Init(AppWindow* app_window,
                                 const AppWindow::CreateParams& create_params) {
   app_window_ = app_window;
   frameless_ = create_params.frame == AppWindow::FRAME_NONE;
-  transparent_background_ = create_params.transparent_background;
   resizable_ = create_params.resizable;
   size_constraints_.set_minimum_size(
       create_params.GetContentMinimumSize(gfx::Insets()));
@@ -260,7 +258,7 @@ void NativeAppWindowViews::OnWidgetActivationChanged(views::Widget* widget,
 
 void NativeAppWindowViews::RenderViewCreated(
     content::RenderViewHost* render_view_host) {
-  if (transparent_background_) {
+  if (app_window_->RequestedTransparentBackground() && CanHaveAlphaEnabled()) {
     content::RenderWidgetHostView* view = render_view_host->GetView();
     DCHECK(view);
     view->SetBackgroundOpaque(false);
@@ -398,4 +396,12 @@ void NativeAppWindowViews::SetContentSizeConstraints(
   size_constraints_.set_maximum_size(max_size);
 }
 
+bool NativeAppWindowViews::CanHaveAlphaEnabled() const {
+  return true;

[tapted, 2014/07/14 03:30:24]

does this work fine on desktop-linux as well as ChromeOS? I thought we were
still missing things for the general case there..

[jackhou1, 2014/07/14 04:51:02]

This doesn't work on Linux. 

I think this logic should live in the tree host, but I'm not sure what's the
right way to pipe it back up to the NativeAppWindow. What's in this patch set
obviously doesn't work though, so I'll implement it in a way that works (add
CanHaveAlphaEnabled to Widget and WindowTreeHost) and wait for benwells'
comments.

+}
+
+void NativeAppWindowViews::CanHaveAlphaEnabledChanged() {
+  app_window_->OnNativeWindowChanged();
+}
+
 }  // namespace apps