Lots of white space at the bottom on `google.com.tw/search`

[tigercosmos]

build 64c514a with macOS
probably related to #16185

[pyfisch]

Some notes:

• There is also a lot of white space on the Google home page e.g. https://google.com
• The scrollable area is set of the root element is set in layout_thread
  • In the if/else construct wherever I checked the else branch is taken
• If you fix this you should try if https://xkcd.com/ still works (because the page scrolls the body instead of root?)
• The function setting the root element size has a TODO by @pcwalton saying that it does not conform to the spec.