Tweak priorities of paint invalidation reasons

third_party/WebKit/LayoutTests/fast/repaint/subtree-root-clip-expected.txt

 {
   "name": "Content Root Layer",
   "bounds": [800, 600],
   "children": [
     {
       "name": "LayoutView #document",
       "bounds": [800, 600],
       "contentsOpaque": true,
       "drawsContent": true,
       "paintInvalidations": [
         {
           "object": "LayoutBlockFlow (floating) DIV",
           "rect": [8, 8, 100, 100],
           "reason": "background obscuration change"
         },
         {
           "object": "LayoutBlockFlow DIV id='inner'",
           "rect": [8, 8, 100, 100],
-          "reason": "bounds change"
+          "reason": "became invisible"
         }
       ]
     }
   ],
   "objectPaintInvalidations": [
     {
       "object": "LayoutBlockFlow (floating) DIV",
       "reason": "background obscuration change"
     },
     {
       "object": "LayoutBlockFlow DIV id='inner'",
-      "reason": "bounds change"
+      "reason": "became invisible"
     }
   ]
 }