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

LayoutTests/compositing/squashing/clip-composited-descendant-expected.txt

Index: LayoutTests/compositing/squashing/clip-composited-descendant-expected.txt
diff --git a/LayoutTests/compositing/squashing/repaint-child-of-squashed-expected.txt b/LayoutTests/compositing/squashing/clip-composited-descendant-expected.txt
similarity index 72%
copy from LayoutTests/compositing/squashing/repaint-child-of-squashed-expected.txt
copy to LayoutTests/compositing/squashing/clip-composited-descendant-expected.txt
index 012912ca78c5e068010c4c44d9fe91e0e17ab1b7..88c1bfa71d5156debbf4943feec27a6289bb2f95 100644
--- a/LayoutTests/compositing/squashing/repaint-child-of-squashed-expected.txt
+++ b/LayoutTests/compositing/squashing/clip-composited-descendant-expected.txt
@@ -17,8 +17,8 @@ CASE 1, original layer tree
               (backgroundColor #0000FF)
             )
             (GraphicsLayer
-              (position 130.00 130.00)
-              (bounds 100.00 100.00)
+              (position 100.00 20.00)
+              (bounds 130.00 210.00)
               (drawsContent 1)
             )
           )
@@ -27,7 +27,7 @@ CASE 1, original layer tree
     )
   )
 )
-CASE 2, change color of "inner" to red
+CASE 2, layer tree with overflow:hidden on the parent of a composited box. Parent is no longer squashed.
 (GraphicsLayer
   (bounds 800.00 600.00)
   (children 1
@@ -35,7 +35,7 @@ CASE 2, change color of "inner" to red
       (bounds 800.00 600.00)
       (contentsOpaque 1)
       (drawsContent 1)
-      (children 1
+      (children 2
         (GraphicsLayer
           (children 2
             (GraphicsLayer
@@ -46,16 +46,19 @@ CASE 2, change color of "inner" to red
               (backgroundColor #0000FF)
             )
             (GraphicsLayer
-              (position 130.00 130.00)
+              (position 100.00 20.00)
               (bounds 100.00 100.00)
               (drawsContent 1)
-              (repaint rects
-                (rect 10.00 10.00 50.00 50.00)
-                (rect 10.00 10.00 50.00 50.00)
-              )
             )
           )
         )
+        (GraphicsLayer
+          (position 130.00 130.00)
+          (bounds 100.00 100.00)
+          (contentsOpaque 1)
+          (drawsContent 1)
+          (backgroundColor #00FFFF)
+        )
       )
     )
   )