This is a follow-up from issue #1539.

If the currently selected browsing context is already the top-level browsing context the call to "Switch To Parent Frame" should be a no-op. That's how the Chromedriver behaves currently. But right now when reading the spec the first step of the remote steps will actually error out:

https://w3c.github.io/webdriver/#switch-to-parent-frame

If the current parent browsing context is no longer open, return error with error code no such window.

If the current browsing context is already the top browsing context, the parent will be null. As such the command should return a "no such window" error.

CC'ing @jgraham for his opinion.