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.

tests/cefclient/browser/default_client_handler.cc

@@ -4,8 +4,16 @@
 
 #include "tests/cefclient/browser/default_client_handler.h"
 
+#include "tests/cefclient/browser/main_context.h"
+#include "tests/cefclient/browser/root_window_manager.h"
+
 namespace client {
 
+DefaultClientHandler::DefaultClientHandler(std::optional<bool> use_alloy_style)
+    : use_alloy_style_(
+          use_alloy_style.value_or(MainContext::Get()->UseAlloyStyleGlobal())) {
+}
+
 // static
 CefRefPtr<DefaultClientHandler> DefaultClientHandler::GetForClient(
     CefRefPtr<CefClient> client) {
@@ -16,4 +24,39 @@ CefRefPtr<DefaultClientHandler> DefaultClientHandler::GetForClient(
   return nullptr;
 }
 
+bool DefaultClientHandler::OnBeforePopup(
+    CefRefPtr<CefBrowser> browser,
+    CefRefPtr<CefFrame> frame,
+    const CefString& target_url,
+    const CefString& target_frame_name,
+    CefLifeSpanHandler::WindowOpenDisposition target_disposition,
+    bool user_gesture,
+    const CefPopupFeatures& popupFeatures,
+    CefWindowInfo& windowInfo,
+    CefRefPtr<CefClient>& client,
+    CefBrowserSettings& settings,
+    CefRefPtr<CefDictionaryValue>& extra_info,
+    bool* no_javascript_access) {
+  CEF_REQUIRE_UI_THREAD();
+
+  if (target_disposition == CEF_WOD_NEW_PICTURE_IN_PICTURE) {
+    // Use default handling for document picture-in-picture popups.
+    client = nullptr;
+    return false;
+  }
+
+  // Used to configure default values.
+  RootWindowConfig config(/*command_line=*/nullptr);
+
+  // Potentially create a new RootWindow for the popup browser that will be
+  // created asynchronously.
+  MainContext::Get()->GetRootWindowManager()->CreateRootWindowAsPopup(
+      config.use_views, use_alloy_style_, config.with_controls,
+      /*is_osr=*/false, /*is_devtools=*/false, popupFeatures, windowInfo,
+      client, settings);
+
+  // Allow popup creation.
+  return false;
+}
+
 }  // namespace client