Eagerly invalidate when a layer becomes stacking context

third_party/WebKit/LayoutTests/paint/invalidation/css-grid-layout/grid-item-z-index-change-repaint-expected.txt

 {
   "layers": [
     {
       "name": "LayoutView #document",
       "bounds": [800, 600],
       "contentsOpaque": true,
       "drawsContent": true,
       "paintInvalidations": [
         {
           "object": "LayoutBlockFlow DIV id='item2' class='sizedToGridArea green'",
           "rect": [8, 236, 200, 100],
           "reason": "subtree"
         },
         {
+          "object": "LayoutBlockFlow DIV id='item2' class='sizedToGridArea green'",
+          "rect": [8, 236, 200, 100],
+          "reason": "subtree"
+        },
+        {
           "object": "LayoutBlockFlow DIV id='item1' class='sizedToGridArea green negativeZIndex'",
           "rect": [8, 126, 200, 100],
           "reason": "subtree"
         }
       ]
     }
   ],
   "objectPaintInvalidations": [
     {
+      "object": "LayoutBlockFlow DIV id='item2' class='sizedToGridArea green'",
+      "reason": "subtree"
+    },
+    {
       "object": "LayoutBlockFlow DIV id='item1' class='sizedToGridArea green negativeZIndex'",
       "reason": "subtree"
     },
     {
       "object": "LayoutBlockFlow DIV id='item2' class='sizedToGridArea green'",
       "reason": "subtree"
     }
   ]
 }