cefclient: views: Add overlay BrowserView demo (see #3790)

Adds new `--show-overlay-browser` command-line flag that creates an
overlay containing an Alloy-style BrowserView.  The main BrowserView
may be Chrome- (default) or Alloy-style (add `--use-alloy-style`).
This overlay will only be added to standalone (non-popup) windows.

Popups triggered via the overlay will receive standard Alloy-style
handling in cefclient (e.g. Views-hosted window). Add
`--use-default-popup` for default popup handling.

libcef/browser/views/browser_view_impl.cc

@@ -280,8 +280,8 @@ void CefBrowserViewImpl::AddedToWidget() {
   CefWidget* cef_widget = CefWidget::GetForWidget(widget);
   DCHECK(cef_widget);
 
-  if (!browser_) {
-    if (cef_widget->IsAlloyStyle() && !is_alloy_style_) {
+  if (!browser_ && !is_alloy_style_) {
+    if (cef_widget->IsAlloyStyle()) {
       LOG(ERROR) << "Cannot add Chrome style BrowserView to Alloy style Window";
       return;
     }