In the upcoming change, device pixel conversion of ClipFrame will
happen during display list replay, where PaintContext is not available,
so let’s move it out of PaintContext.
We previously checked the cell's computed values for the border-collapse
property, but a cell is only in separated-borders mode if the table has
border-collapse set to separate. Curiously in some other placed where
this mode is checked we already did the correct thing.
This reduces the number of `.cpp` files that need to be recompiled when
one of the below header files changes as follows:
Painting/Command.h: 1030 -> 61
Painting/DisplayList.h: 1030 -> 60
Painting/DisplayListRecorder.h: 557 -> 59
