Bug 1638594 - Handle layout coordinates relative to the RCD viewport frame correctly in the GetECRT slow-path. r=tnikkel, a=jcristau

Asking for layout coordinates relative to the RCD viewport frame
is a special case because the RCD viewport frame is outside the
zoom boundary, and so should technically always use visual
coordinates.

Nonetheless, we need to use layout coordinates relative to the
RCD viewport frame in several places, as we don't have a frame
that's inside the zoom boundary and sized to the initial
containing block (bug 1641279 may introduce such a frame in the
future).

This means GetEventCoordinatesRelativeTo needs special handling
to apply the visual-to-layout transform when converting from
visual coordinates relative to the root document's viewport
frame, to layout coordinates relative to the RCD viewport frame.

If the RCD is the root document, we take the fast-path which
handled this with an explicit check, but if the RCD is not the
root document, we take the slow-path which does not handle this.
This patch refactors GetECRT so it always checks for this
special case at the end.

Depends on D79589

Differential Revision: https://phabricator.services.mozilla.com/D79590