Replace coversExtraPixels with simpler logic

third_party/WebKit/LayoutTests/paint/invalidation/resize-with-border-clipped-expected.txt

 {
   "layers": [
     {
       "name": "LayoutView #document",
       "bounds": [800, 600],
       "contentsOpaque": true,
       "drawsContent": true,
       "paintInvalidations": [
         {
           "object": "LayoutBlockFlow DIV id='target'",
-          "rect": [100, 210, 200, 90],
-          "reason": "incremental"
-        },
-        {
-          "object": "LayoutBlockFlow DIV id='target'",
-          "rect": [210, 100, 90, 200],
-          "reason": "incremental"
+          "rect": [100, 100, 200, 200],
+          "reason": "border box change"
         }
       ]
     }
   ],
   "objectPaintInvalidations": [
     {
       "object": "LayoutBlockFlow DIV id='target'",
-      "reason": "incremental"
+      "reason": "border box change"
     }
   ]
 }