Simplify incremental paint invalidation

third_party/WebKit/LayoutTests/paint/invalidation/border-repaint-glitch-expected.txt

 {
   "layers": [
     {
       "name": "LayoutView #document",
       "bounds": [800, 600],
       "contentsOpaque": true,
       "drawsContent": true,
       "paintInvalidations": [
         {
           "object": "LayoutBlockFlow DIV",
           "rect": [19, 401, 200, 24],
           "reason": "incremental"
         },
         {
           "object": "LayoutBlockFlow DIV",
           "rect": [19, 241, 200, 24],
           "reason": "incremental"
         },
         {
-          "object": "LayoutBlockFlow DIV",
-          "rect": [19, 405, 200, 20],
-          "reason": "incremental"
-        },
-        {
-          "object": "LayoutBlockFlow DIV",
-          "rect": [19, 245, 200, 20],
-          "reason": "incremental"
-        },
-        {
           "object": "LayoutBlockFlow DIV id='b'",
           "rect": [33, 391, 172, 20],
           "reason": "incremental"
         },
         {
           "object": "LayoutBlockFlow DIV id='a'",
           "rect": [33, 231, 172, 20],
           "reason": "incremental"
         }
       ]
@@ skipping 12 matching lines @@
       "object": "LayoutBlockFlow DIV",
       "reason": "incremental"
     },
     {
       "object": "LayoutBlockFlow DIV id='b'",
       "reason": "incremental"
     }
   ]
 }