Refactor PLSA scrollbar existence calculation.

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'",
@@ skipping 28 matching lines @@
     },
     {
       "name": "Overflow Controls Host Layer",
       "bounds": [35, 13]
     },
     {
       "name": "Horizontal Scrollbar Layer",
       "position": [0, 13],
       "bounds": [35, 0],
       "drawsContent": true
-    },
-    {
-      "name": "Vertical Scrollbar Layer",
-      "position": [35, 0],
-      "bounds": [0, 13],
-      "drawsContent": true
     }
   ],
   "objectPaintInvalidations": [
     {
       "object": "LayoutBlockFlow HTML",
       "reason": "selection"
     },
     {
       "object": "LayoutBlockFlow BODY",
       "reason": "selection"
@@ skipping 10 matching lines @@
       "object": "LayoutText #text",
       "reason": "selection"
     },
     {
       "object": "InlineTextBox 'test test test'",
       "reason": "selection"
     }
   ]
 }