Don't squash layers that have a clip or overflow clip.

LayoutTests/compositing/fixed-position-changed-to-absolute-expected.txt

Index: LayoutTests/compositing/fixed-position-changed-to-absolute-expected.txt
diff --git a/LayoutTests/compositing/fixed-position-changed-to-absolute-expected.txt b/LayoutTests/compositing/fixed-position-changed-to-absolute-expected.txt
index 2eed79591644c8ebc67b4fce8e95e83d125067e5..b26b47363ecb2aad756400ba8cc231a4452bb52a 100644
--- a/LayoutTests/compositing/fixed-position-changed-to-absolute-expected.txt
+++ b/LayoutTests/compositing/fixed-position-changed-to-absolute-expected.txt
@@ -5,19 +5,26 @@
       (bounds 800.00 600.00)
       (contentsOpaque 1)
       (drawsContent 1)
-      (children 1
+      (children 2
+        (GraphicsLayer
+          (position 8.00 13.00)
+          (bounds 150.00 150.00)
+          (contentsOpaque 1)
+          (drawsContent 1)
+          (backgroundColor #D9CCA7)
+        )
         (GraphicsLayer
           (children 2
             (GraphicsLayer
-              (position 8.00 13.00)
-              (bounds 150.00 150.00)
+              (position 20.00 20.00)
+              (bounds 60.00 60.00)
               (contentsOpaque 1)
               (drawsContent 1)
-              (backgroundColor #D9CCA7)
+              (backgroundColor #6AA6A6)
             )
             (GraphicsLayer
-              (position 20.00 20.00)
-              (bounds 226.00 180.00)
+              (position 40.00 40.00)
+              (bounds 206.00 160.00)
               (drawsContent 1)
             )
           )