CSS Borders don't support track component feature

[shai-almog]

RoundRectBorder uses the getTrackComponent() feature derived from Border to implement an arrow that tracks a component which is used by popup dialog etc. to point at a component.
Unfortunately this feature doesn't work for CSS so if we do something like styling a popup dialog through CSS it either doesn't work at all (no impact) or it just fails very badly and loses the arrow functionality entirely.

CSS needs to add that capability or possibly provide the ability to just use the RoundRectBorder directly as it does with the special case syntax cn1-pill-border.