WebContent: Add shutdown_server IPC call to request process exit

This ensures that removing the last view from a WebContentClient will
close its associated process, assuming the WebContent process is not
hung. A more drastic measure will be needed to trigger forcefully
killing the process when it doesn't respond to this request.

Userland/Libraries/LibWebView/WebContentClient.cpp

@@ -50,6 +50,7 @@ void WebContentClient::unregister_view(u64 page_id)
     m_views.remove(page_id);
     if (m_views.is_empty()) {
         on_web_content_process_crash = nullptr;
+        async_close_server();
     }
 }
 

Userland/Services/WebContent/ConnectionFromClient.cpp

@@ -82,6 +82,11 @@ Optional<PageClient const&> ConnectionFromClient::page(u64 index, SourceLocation
     return {};
 }
 
+void ConnectionFromClient::close_server()
+{
+    shutdown();
+}
+
 Messages::WebContentServer::GetWindowHandleResponse ConnectionFromClient::get_window_handle(u64 page_id)
 {
     if (auto page = this->page(page_id); page.has_value())

Userland/Services/WebContent/ConnectionFromClient.h

@@ -53,6 +53,7 @@ private:
     Optional<PageClient&> page(u64 index, SourceLocation = SourceLocation::current());
     Optional<PageClient const&> page(u64 index, SourceLocation = SourceLocation::current()) const;
 
+    virtual void close_server() override;
     virtual Messages::WebContentServer::GetWindowHandleResponse get_window_handle(u64 page_id) override;
     virtual void set_window_handle(u64 page_id, String const& handle) override;
     virtual void connect_to_webdriver(u64 page_id, ByteString const& webdriver_ipc_path) override;

Userland/Services/WebContent/WebContentServer.ipc

@@ -13,6 +13,8 @@
 
 endpoint WebContentServer
 {
+    close_server() =|
+
     get_window_handle(u64 page_id) => (String handle)
     set_window_handle(u64 page_id, String handle) =|