Tweak priorities of paint invalidation reasons

third_party/WebKit/LayoutTests/fast/repaint/window-resize-background-image-generated-expected.txt

 {
   "name": "Content Root Layer",
   "bounds": [600, 250],
   "children": [
     {
       "name": "LayoutView #document",
       "bounds": [600, 250],
       "contentsOpaque": true,
       "drawsContent": true,
       "paintInvalidations": [
@@ skipping 45 matching lines @@
     }
   ],
   "objectPaintInvalidations": [
     {
       "object": "LayoutView #document",
       "reason": "bounds change"
     },
     {
       "object": "LayoutBlockFlow HTML",
       "reason": "forced by layout"
-    },
-    {
-      "object": "LayoutBlockFlow BODY",
-      "reason": "became visible"
     }
   ]
 }
 {
   "name": "Content Root Layer",
   "bounds": [400, 600],
   "children": [
     {
       "name": "LayoutView #document",
       "bounds": [400, 600],
@@ skipping 48 matching lines @@
     }
   ],
   "objectPaintInvalidations": [
     {
       "object": "LayoutView #document",
       "reason": "bounds change"
     },
     {
       "object": "LayoutBlockFlow HTML",
       "reason": "forced by layout"
-    },
-    {
-      "object": "LayoutBlockFlow BODY",
-      "reason": "became visible"
     }
   ]
 }