[MD] Implement incognito colors as a NativeTheme (for Aura).

ui/views/widget/widget.cc

Index: ui/views/widget/widget.cc
diff --git a/ui/views/widget/widget.cc b/ui/views/widget/widget.cc
index c3c0252a7f91fe0a645dfa7be4deed08055e4bd7..ae755dbd7beb6e211942352ee47abf0f37c511f3 100644
--- a/ui/views/widget/widget.cc
+++ b/ui/views/widget/widget.cc
@@ -766,6 +766,12 @@ ui::ThemeProvider* Widget::GetThemeProvider() const {
 }
 
 const ui::NativeTheme* Widget::GetNativeTheme() const {
+  if (ViewsDelegate::GetInstance()) {
+    ui::NativeTheme* theme =
+        ViewsDelegate::GetInstance()->GetNonDefaultNativeTheme(this);
+    if (theme)
+      return theme;
+  }
   return native_widget_->GetNativeTheme();
 }