Clear the groupedMapping parameter in layers when removed from the group

LayoutTests/compositing/squashing/add-remove-squashed-layers-expected.txt

 Test that layers can be nicely added or removed from a squashed layer, without unnecessary repaints on any layer. Click anywhere to test interactively; keep clicking to proceed through the test.
 
 CASE 1, original layer tree with overlap1 and overlap2:
 {
   "bounds": [800, 600],
   "children": [
     {
       "bounds": [800, 600],
       "contentsOpaque": true,
       "drawsContent": true,
@@ skipping 121 matching lines @@
   ]
 }
 CASE 5, overlap2 gets added back:
 {
   "bounds": [800, 600],
   "children": [
     {
       "bounds": [800, 600],
       "contentsOpaque": true,
       "drawsContent": true,
+      "repaintRects": [
+        [0, 0, 100, 100]
+      ],
       "children": [
         {
           "shouldFlattenTransform": false,
           "children": [
             {
               "position": [60, 60],
               "bounds": [400, 400],
               "contentsOpaque": true,
               "drawsContent": true,
               "backgroundColor": "#808080"
             },
             {
               "position": [220, 220],
               "bounds": [180, 180],
               "drawsContent": true,
               "repaintRects": [
                 [80, 80, 100, 100],
-                [0, 0, 100, 100],
                 [0, 0, 100, 100]
               ]
             }
           ]
         }
       ]
     }
   ]
 }
 CASE 6, overlap1 gets added back, and overlap3 gets removed:
 {
   "bounds": [800, 600],
   "children": [
     {
       "bounds": [800, 600],
       "contentsOpaque": true,
       "drawsContent": true,
+      "repaintRects": [
+        [0, 0, 100, 100]
+      ],
       "children": [
         {
           "shouldFlattenTransform": false,
           "children": [
             {
               "position": [60, 60],
               "bounds": [400, 400],
               "contentsOpaque": true,
               "drawsContent": true,
               "backgroundColor": "#808080"
             },
             {
               "position": [140, 140],
               "bounds": [180, 180],
               "drawsContent": true,
               "repaintRects": [
                 [80, 80, 100, 100],
                 [80, 80, 100, 100],
-                [0, 0, 100, 100],
                 [0, 0, 100, 100]
               ]
             }
           ]
         }
       ]
     }
   ]
 }