`/webdriver/tests/classic/take_screenshot/iframe.py`, `/webdriver/tests/classic/take_element_screenshot/iframe.py` intermittently captures a screenshot of empty page

It's always the reference screenshot, for some reason. The webdriver session loads a page containing an iframe, then blocks on the page being fully loaded, then takes a screenshot of the top level browsing context. The test passes when this screenshot contains real pixels, and fails when the screenshot contains only white pixels.