Set scrollbar enabled state for overflow:auto.

third_party/WebKit/LayoutTests/paint/invalidation/flexbox/scrollbars-changed-expected.txt

 {
   "layers": [
     {
       "name": "LayoutView #document",
       "bounds": [800, 600],
       "contentsOpaque": true,
       "drawsContent": true
     },
     {
       "name": "LayoutBlockFlow DIV id='scroller'",
@@ skipping 39 matching lines @@
           "rect": [0, 0, 15, 210],
           "reason": "full"
         },
         {
           "object": "Vertical Scrollbar Layer",
           "rect": [0, 0, 15, 210],
           "reason": "full"
         },
         {
           "object": "Vertical Scrollbar Layer",
+          "rect": [0, 0, 15, 210],
+          "reason": "full"

[bokan, 2016/11/17 17:04:45]

The extra invalidations come from changing the enabled state on an overflow:auto
scrollbar, which didn't happen before. This shouldn't matter though because if
we've changed the enabled state, the scrollbar is about to be removed which will
cause an invalidation anyway.

[skobes, 2016/11/17 21:18:23]

Acknowledged.

+        },
+        {
+          "object": "Vertical Scrollbar Layer",
           "rect": [0, 0, 15, 100],
           "reason": "full"
         },
+        {
+          "object": "Vertical Scrollbar Layer",
+          "rect": [0, 0, 15, 100],
+          "reason": "full"
+        },
         {
           "object": "Vertical Scrollbar Layer",
           "rect": [0, 0, 15, 100],
           "reason": "full"
         },
         {
           "object": "Vertical Scrollbar Layer",
           "rect": [0, 0, 15, 100],
           "reason": "full"
         },
@@ skipping 18 matching lines @@
       "object": "LayoutText #text",
       "reason": "full"
     },
     {
       "object": "InlineTextBox 'z'",
       "reason": "full"
     }
   ]
 }