WebDriver: Rename Session::{get_window_object => current_window}()

- It's not a "window object" (in the JS sense), it's a simple struct
  storing a handle and is_open property
- We usually omit 'get' for getters
- The new name makes it more clear that this is looked up using the
  m_current_window_handle as a key

Userland/Services/WebDriver/Session.h

@@ -33,7 +33,7 @@ public:
         i32 id;
     };
 
-    Optional<Window*> get_window_object() { return m_windows.get(m_current_window_handle); }
+    Optional<Window*> current_window() { return m_windows.get(m_current_window_handle); }
     String const& current_window_handle() { return m_current_window_handle; }
 
     ErrorOr<void> start();