Remove getPageLayoutBox, getLayoutBox, and getIntersectionChangeEntry APIs from docking

[dvoytenko]

Partial for #31540.

A more holistic rewrite could eliminate significant amount of measures. However, in my analysis:

1. Most of the measurements (and most performance sensitive ones) happen as the result of PositionObserver callback. However, that callback, conveniently, provides the ClientRect structure, which I used when possible to eliminate most of the measurements.
2. The code as it stands is fairly efficient. E.g. it starts monitoring of positions only when the element is in a right corridor.

[samouri]

Assuming you've tested the functionality, code LGTM