Setting the caret via accessibility should scroll content into view

[Samuel Thibault <sthibault@debian.org>]

Hello,

For information, this bug is fixed in firefox 58, available in beta
version in experimental, so people might want to already pick it up from
there.

> Impact: An AT that controls navigation (like the Orca screen reader) is
> often presenting content which is not visible -- no problem for a user who
> is blind and working alone. But if the Orca user is working with a sighted
> peer, or using Orca along with magnification software which tracks the caret
> location, that peer or low vision user cannot see what is being presented."

Samuel