Add app.window.alphaEnabled() and onAlphaEnabledChanged.

ui/views/widget/desktop_aura/desktop_window_tree_host_win.cc

Index: ui/views/widget/desktop_aura/desktop_window_tree_host_win.cc
diff --git a/ui/views/widget/desktop_aura/desktop_window_tree_host_win.cc b/ui/views/widget/desktop_aura/desktop_window_tree_host_win.cc
index fc2bda6311aa5a4f12320aa4e32dddff7e74c827..634752678154fddcd183b1a60cfb11e7aea365c6 100644
--- a/ui/views/widget/desktop_aura/desktop_window_tree_host_win.cc
+++ b/ui/views/widget/desktop_aura/desktop_window_tree_host_win.cc
@@ -365,7 +365,7 @@ void DesktopWindowTreeHostWin::SetVisibilityChangedAnimationsEnabled(
 }
 
 bool DesktopWindowTreeHostWin::ShouldUseNativeFrame() const {
-  return ui::win::IsAeroGlassEnabled();
+  return IsTranslucentWindowOpacitySupported();
 }
 
 bool DesktopWindowTreeHostWin::ShouldWindowContentsBeTransparent() const {
@@ -427,6 +427,10 @@ bool DesktopWindowTreeHostWin::IsAnimatingClosed() const {
   return pending_close_;
 }
 
+bool DesktopWindowTreeHostWin::IsTranslucentWindowOpacitySupported() const {
+  return ui::win::IsAeroGlassEnabled();
+}
+
 ////////////////////////////////////////////////////////////////////////////////
 // DesktopWindowTreeHostWin, WindowTreeHost implementation: