We try to vertically center the diff collapse button in the viewable

content area of an expanded region, but there were some issues with the

logic:


If there's a banner, it reduces the available content area. It's then

   possible for the button to appear below the banner. This is the case

   with the Unified Banner.


The button only vertically centered in relation to the

   expanded/collapsed tbody. Ajacent tbodies of the same type (e.g.,

   "equals") weren't considered, and this made the button appear shifted

   up or down.


The button didn't smoothly jump to new positions until it reached

   fixed position mode (which is when we know we're centering it

   absolutely vertically in the viewport). This had a pretty bad feel to

   it.


The new design fixes all of these issues.
We now factor in the new content viewport to shift the available space

by the Unified Banner's height, ensuring there's never an overlap.
We can now specify a top, bottom, and parent for the element when

centering, offsetting the position as appropriate. DiffReviewableView

will set these based on the types of adjacent tbodies. This gives us teh

entire span of lines of a type as the boundaries for the centering.
Position updates are smoother, now using requestAnimation() rather

than a delay for repositioning. We also avoid setting more CSS

properties than we need to at once, and we do a better job of capping

the button in the contained area.

Unit tests pass.
Tested fixed-based vertical centering when the viewport spans an entire

expanded area.
Tested absolute-based vertical centering at the top and bottom of an

expanded area.
Tested that it stops at the top-most and bottom-most boundaries and does

not overflow.
Tested that it adjusts based on the scrolling of the Unified Banner.
Tested that it appears smooth and not jumpy in all cases.