Fix issue when background changes painting target layer

third_party/WebKit/LayoutTests/paint/invalidation/compositing/invalidate-when-leaving-squashed-layer-expected.txt

 {
   "layers": [
     {
       "name": "LayoutView #document",
       "bounds": [800, 600],
       "contentsOpaque": true,
       "drawsContent": true
     },
     {
       "name": "Squashing Containment Layer",
@@ skipping 24 matching lines @@
       "name": "LayoutBlockFlow (positioned) DIV id='target'",
       "position": [50, 50],
       "bounds": [200, 200],
       "contentsOpaque": true,
       "drawsContent": true,
       "backgroundColor": "#ADD8E6",
       "paintInvalidations": [
         {
           "object": "LayoutBlockFlow (positioned) DIV id='target'",
           "rect": [0, 0, 200, 200],
+          "reason": "full"
+        },
+        {
+          "object": "LayoutBlockFlow (positioned) DIV id='target'",
+          "rect": [0, 0, 200, 200],
           "reason": "subtree"
         }
       ]
     }
   ],
   "objectPaintInvalidations": [
     {
       "object": "LayoutBlockFlow (positioned) DIV id='target'",
       "reason": "subtree"
     },
     {
       "object": "LayoutBlockFlow (positioned) DIV id='target'",
       "reason": "subtree"
     }
   ]
 }