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
