Consider merging https://chromium.googlesource.com/chromium/blink/+/4cb65a17e49d135e7d0d4209eb557e98b2d7b097
Document::setBody previously would create a deep copy of the node if you moved it
from a different document and didn't allow framesets. Document::body would incorrectly
return <frameset> even if <body> came first. I also made setBody AttachLazily just
like doing appendChild(body) would have done.
Fixed the getter via Bug 159488.
Fixing the setter via Bug 159490.