This makes it so that view-transition pseudo-element styles are
updated before returning them from window.getComputedStyle(). This
is necessary because they could be outdated, in case JS has
modified the styles of the elements they are trying to stay in sync
with since last frame.
The corresponding WPT test has not been imported, since it still
fails for unrelated reasons.
This also fixes a bug in the view transitions code that was
required to get the imported test to pass. The code for setting
the initial containing block size just did not set the right thing,
since doing so would trigger an error later on.
That later error resulted from walking up the tree, without
considering that the document element has a parent that is not
itself an element. (and then doing element things to it)
