This code dates back to the time when absolutely positioned flows were
ignored by all of their ancestors up to the containing block. This
hasn't been true for at least a year.
Closes#9306.
Closes#9309.
Is a partial fix for #9308.
r? @glennw
Source-Repo: https://github.com/servo/servo
Source-Revision: 4755cc5c6211e845fc8081cb3c8a67b4bdbea5cc