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 11717d3b6bf6358e4ae03dfc879aa74b303d9596..acbef2b60820a32f2e22a45f852d0d75eb59f666 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: