WebContent+WebDriver: Set the navigator.webdriver flag from WebDriver

This moves setting the navigator.webdriver flag from after WebContent
creates the WebDriver connection, to its own IPC to be triggered from
WebDriver. This is closer to the spec, but mostly serves as an easy
test to validate the connection.

Userland/Services/WebContent/WebDriverConnection.h

@@ -28,6 +28,7 @@ private:
     WebDriverConnection(NonnullOwnPtr<Core::Stream::LocalSocket> socket, PageHost& page_host);
 
     virtual void die() override { }
+    virtual void set_is_webdriver_active(bool) override;
 
     PageHost& m_page_host;
 };