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.h

@@ -6,6 +6,8 @@
 #define CEF_TESTS_CEFCLIENT_BROWSER_DEFAULT_CLIENT_HANDLER_H_
 #pragma once
 
+#include <optional>
+
 #include "tests/cefclient/browser/base_client_handler.h"
 
 namespace client {
@@ -14,18 +16,37 @@ namespace client {
 // style only.
 class DefaultClientHandler : public BaseClientHandler {
  public:
-  DefaultClientHandler() = default;
+  // If |use_alloy_style| is nullopt the global default will be used.
+  explicit DefaultClientHandler(
+      std::optional<bool> use_alloy_style = std::nullopt);
 
   // Returns the DefaultClientHandler for |client|, or nullptr if |client| is
   // not a DefaultClientHandler.
   static CefRefPtr<DefaultClientHandler> GetForClient(
       CefRefPtr<CefClient> client);
 
+ protected:
+  bool 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) override;
+
  private:
   // Used to determine the object type.
   virtual const void* GetTypeKey() const override { return &kTypeKey; }
   static constexpr int kTypeKey = 0;
 
+  const bool use_alloy_style_;
+
   IMPLEMENT_REFCOUNTING(DefaultClientHandler);
   DISALLOW_COPY_AND_ASSIGN(DefaultClientHandler);
 };