Clear the groupedMapping parameter in layers when removed from the group

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

Index: LayoutTests/compositing/squashing/add-remove-squashed-layers-expected.txt
diff --git a/LayoutTests/compositing/squashing/add-remove-squashed-layers-expected.txt b/LayoutTests/compositing/squashing/add-remove-squashed-layers-expected.txt
index 49a7939d101ff79cbef894501a11171e49cc9dd1..30bd9622df8955791a7e41bb44b359ee4c3a4435 100644
--- a/LayoutTests/compositing/squashing/add-remove-squashed-layers-expected.txt
+++ b/LayoutTests/compositing/squashing/add-remove-squashed-layers-expected.txt
@@ -139,6 +139,9 @@ CASE 5, overlap2 gets added back:
       "bounds": [800, 600],
       "contentsOpaque": true,
       "drawsContent": true,
+      "repaintRects": [
+        [0, 0, 100, 100]
+      ],
       "children": [
         {
           "shouldFlattenTransform": false,
@@ -156,7 +159,6 @@ CASE 5, overlap2 gets added back:
               "drawsContent": true,
               "repaintRects": [
                 [80, 80, 100, 100],
-                [0, 0, 100, 100],
                 [0, 0, 100, 100]
               ]
             }
@@ -174,6 +176,9 @@ CASE 6, overlap1 gets added back, and overlap3 gets removed:
       "bounds": [800, 600],
       "contentsOpaque": true,
       "drawsContent": true,
+      "repaintRects": [
+        [0, 0, 100, 100]
+      ],
       "children": [
         {
           "shouldFlattenTransform": false,
@@ -192,7 +197,6 @@ CASE 6, overlap1 gets added back, and overlap3 gets removed:
               "repaintRects": [
                 [80, 80, 100, 100],
                 [80, 80, 100, 100],
-                [0, 0, 100, 100],
                 [0, 0, 100, 100]
               ]
             }