Add app.window.alphaEnabled() and onAlphaEnabledChanged.

ui/views/widget/native_widget_mac.mm

Index: ui/views/widget/native_widget_mac.mm
diff --git a/ui/views/widget/native_widget_mac.mm b/ui/views/widget/native_widget_mac.mm
index f45510ccd7ff9f611627def6d960f1e2f9f342f1..b76aa48dd7637626deeae9bb1210f6d9e0fa9aba 100644
--- a/ui/views/widget/native_widget_mac.mm
+++ b/ui/views/widget/native_widget_mac.mm
@@ -422,6 +422,10 @@ void NativeWidgetMac::OnRootViewLayout() const {
   NOTIMPLEMENTED();
 }
 
+bool NativeWidgetMac::IsTranslucentWindowOpacitySupported() const {

[sky, 2014/07/17 15:52:32]

Does mac really not support translucent windows?

[tapted, 2014/07/18 00:22:28]

It does, but the ubercompositor setup that will be used on mac currently doesn't
- I'm still tracking down where the alpha channel is getting lost [WIP:
crrev.com/283053002]. We will also need to ensure that the way transparent-pixel
hit-testing is accomplished on Mac ticks all the security boxes before allowing
packaged AppWindows to use it.

+  return false;
+}
+
 void NativeWidgetMac::RepostNativeEvent(gfx::NativeEvent native_event) {
   NOTIMPLEMENTED();
 }