Don't squash fixed-position elements into another fixed-position element that has a clip.

LayoutTests/compositing/squashing/no-squashing-into-fixed-position-that-clips-expected.txt

Index: LayoutTests/compositing/squashing/no-squashing-into-fixed-position-that-clips-expected.txt
diff --git a/LayoutTests/platform/mac/compositing/squashing/no-squashing-into-another-clip-layer-expected.txt b/LayoutTests/compositing/squashing/no-squashing-into-fixed-position-that-clips-expected.txt
similarity index 62%
copy from LayoutTests/platform/mac/compositing/squashing/no-squashing-into-another-clip-layer-expected.txt
copy to LayoutTests/compositing/squashing/no-squashing-into-fixed-position-that-clips-expected.txt
index 0ccd7d78e25919f851115b1846b9d50c78534a4a..e6d6ddcb3f35d6699bba5cb7af7f344892a4bd89 100644
--- a/LayoutTests/platform/mac/compositing/squashing/no-squashing-into-another-clip-layer-expected.txt
+++ b/LayoutTests/compositing/squashing/no-squashing-into-fixed-position-that-clips-expected.txt
@@ -7,30 +7,30 @@
       "drawsContent": true,
       "children": [
         {
-          "position": [8, 8],
-          "bounds": [784, 10],
+          "position": [0, 50],
+          "bounds": [800, 550],
           "contentsOpaque": true,
-          "drawsContent": true,
           "backgroundColor": "#ADD8E6",
           "children": [
             {
-              "bounds": [784, 10],
+              "bounds": [800, 550],
               "children": [
                 {
-                  "bounds": [784, 10],
+                  "position": [400, -10],
+                  "bounds": [24, 100],
                   "contentsOpaque": true,
                   "drawsContent": true,
-                  "backgroundColor": "#F5F5F5"
+                  "backgroundColor": "#D3D3D3"
                 }
               ]
             }
           ]
         },
         {
-          "position": [8, 0],
-          "bounds": [216, 100],
+          "bounds": [800, 60],
+          "contentsOpaque": true,
           "drawsContent": true,
-          "backgroundColor": "#90EE90"
+          "backgroundColor": "#008000"
         }
       ]
     }