WebDriver: Specify callbacks for clients to launch browser windows

This moves the actual launching of browser windows to the WebDriver main
file. This will allow Ladybird to specify its own callback and re-use
Serenity's Session class.

Userland/Services/WebDriver/Session.cpp

@@ -58,33 +58,17 @@ ErrorOr<NonnullRefPtr<Core::LocalServer>> Session::create_server(DeprecatedStrin
     return server;
 }
 
-ErrorOr<void> Session::start()
+ErrorOr<void> Session::start(LaunchBrowserCallbacks const& callbacks)
 {
     auto promise = TRY(ServerPromise::try_create());
 
     auto web_content_socket_path = DeprecatedString::formatted("{}/webdriver/session_{}_{}", TRY(Core::StandardPaths::runtime_directory()), getpid(), m_id);
     auto web_content_server = TRY(create_server(web_content_socket_path, promise));
 
-    if (m_options.headless) {
-        char const* argv[] = {
-            "/bin/headless-browser",
-            "--webdriver-ipc-path",
-            web_content_socket_path.characters(),
-            "about:blank",
-            nullptr,
-        };
-
-        m_browser_pid = TRY(Core::System::posix_spawn("/bin/headless-browser"sv, nullptr, nullptr, const_cast<char**>(argv), environ));
-    } else {
-        char const* argv[] = {
-            "/bin/Browser",
-            "--webdriver-content-path",
-            web_content_socket_path.characters(),
-            nullptr,
-        };
-
-        m_browser_pid = TRY(Core::System::posix_spawn("/bin/Browser"sv, nullptr, nullptr, const_cast<char**>(argv), environ));
-    }
+    if (m_options.headless)
+        m_browser_pid = TRY(callbacks.launch_headless_browser(web_content_socket_path));
+    else
+        m_browser_pid = TRY(callbacks.launch_browser(web_content_socket_path));
 
     // FIXME: Allow this to be more asynchronous. For now, this at least allows us to propagate
     //        errors received while accepting the Browser and WebContent sockets.