Support testing find-in-page

[josepharhar]

In chromium's content_shell, we can test find-in-page with testRunner.findString. We need to be able to test find-in-page for new features like this one, and it would be nice for WPT to support a way to trigger find-in-page.

We will likely add the new feature to window.find just to make it testable, but window.find isn't specced yet and isn't exactly the same as find-in-page, so it would still be nice to have WPT support for find-in-page as well.

cc @emilio @domenic @annevk