Use the pixelSnappedScrollWidth/Height for contentsSize

third_party/WebKit/LayoutTests/platform/mac/paint/invalidation/selection/selection-in-composited-scrolling-container-expected.txt

 {
   "layers": [
     {
       "name": "LayoutView #document",
       "bounds": [800, 600],
       "contentsOpaque": true,
       "drawsContent": true,
       "paintInvalidations": [
         {
           "object": "LayoutTextControl INPUT id='target'",
           "rect": [5, 5, 47, 25],
           "reason": "subtree"
         }
       ]
     },
     {
       "name": "LayoutBlockFlow DIV id='inner-editor'",
       "position": [11, 11],
       "bounds": [35, 13],
       "shouldFlattenTransform": false,
       "drawsContent": true
     },
     {
       "name": "Scrolling Layer",
       "bounds": [35, 13],
       "shouldFlattenTransform": false
     },
     {
       "name": "Scrolling Contents Layer",
-      "bounds": [66, 13],
+      "bounds": [67, 13],
       "drawsContent": true,
       "paintInvalidations": [
         {
           "object": "LayoutText #text",
           "rect": [23, 0, 23, 13],
           "reason": "selection"
         }
       ]
     },
     {
@@ skipping 34 matching lines @@
       "object": "LayoutText #text",
       "reason": "selection"
     },
     {
       "object": "InlineTextBox 'test test test'",
       "reason": "selection"
     }
   ]
 }