Don't squash layers that have filters.

LayoutTests/compositing/filters/sw-nested-shadow-overlaps-hw-nested-shadow-expected.txt

Index: LayoutTests/compositing/filters/sw-nested-shadow-overlaps-hw-nested-shadow-expected.txt
diff --git a/LayoutTests/compositing/filters/sw-nested-shadow-overlaps-hw-nested-shadow-expected.txt b/LayoutTests/compositing/filters/sw-nested-shadow-overlaps-hw-nested-shadow-expected.txt
index 6e44c215cab88e33ad166856e688d7c408c44b0e..db2d1e68d311ffa9fa1a722f73124253359eafca 100644
--- a/LayoutTests/compositing/filters/sw-nested-shadow-overlaps-hw-nested-shadow-expected.txt
+++ b/LayoutTests/compositing/filters/sw-nested-shadow-overlaps-hw-nested-shadow-expected.txt
@@ -7,19 +7,16 @@
       "drawsContent": true,
       "children": [
         {
-          "children": [
-            {
-              "position": [205, 205],
-              "transformOrigin": [175, 175],
-              "bounds": [225, 225],
-              "drawsContent": true,
-              "backgroundColor": "#000000"
-            },
-            {
-              "bounds": [225, 225],
-              "drawsContent": true
-            }
-          ]
+          "position": [205, 205],
+          "transformOrigin": [175, 175],
+          "bounds": [225, 225],
+          "drawsContent": true,
+          "backgroundColor": "#000000"
+        },
+        {
+          "bounds": [225, 225],
+          "drawsContent": true,
+          "backgroundColor": "#008000"
         }
       ]
     }