Let collapsed border drawings be cacheable

LayoutTests/fast/table/border-collapsing/cached-change-cell-border-width-expected.txt

 {
   "bounds": [800, 600],
   "children": [
     {
       "bounds": [800, 600],
       "contentsOpaque": true,
       "drawsContent": true,
       "repaintRects": [
         [117, 8, 4, 54],
         [116, 8, 5, 54],
         [63, 8, 58, 54],
         [62, 8, 55, 52],
         [10, 10, 110, 50],
         [10, 10, 110, 50],
         [9, 9, 107, 50],
         [9, 9, 107, 50],
         [8, 60, 113, 2],
         [8, 58, 113, 4],
         [8, 8, 59, 54],
         [8, 8, 55, 52]
       ],
       "paintInvalidationClients": [
+        "LayoutTableCell TD id='foo'",

[pdr., 2015/08/20 07:07:15]

Why do we get two invalidations for foo instead of one?

[Xianzhu, 2015/08/20 16:39:51]

The new one is because of change of collapsed border. The other one is because
of other reasons, such as layout, other style change, etc.

+        "LayoutTableCell TD",
         "LayoutTable TABLE",
         "LayoutTableSection TBODY",
         "LayoutTableRow TR",
         "LayoutTableCell TD id='foo'",
         "LayoutTableCell TD"
       ]
     }
   ]
 }