Add a number of tests for layer squashing

LayoutTests/compositing/squashing/squash-three-layers-expected.txt

Index: LayoutTests/compositing/squashing/squash-three-layers-expected.txt
diff --git a/LayoutTests/compositing/squashing/squash-three-layers-expected.txt b/LayoutTests/compositing/squashing/squash-three-layers-expected.txt
new file mode 100644
index 0000000000000000000000000000000000000000..fe82752c9085f71962a92db3cf7ce501c3a64a5a
--- /dev/null
+++ b/LayoutTests/compositing/squashing/squash-three-layers-expected.txt
@@ -0,0 +1,27 @@
+(GraphicsLayer
+  (bounds 800.00 600.00)
+  (children 1
+    (GraphicsLayer
+      (bounds 800.00 600.00)
+      (contentsOpaque 1)
+      (drawsContent 1)
+      (children 1
+        (GraphicsLayer
+          (children 2
+            (GraphicsLayer
+              (position 100.00 100.00)
+              (bounds 100.00 100.00)
+              (contentsOpaque 1)
+            )
+            (GraphicsLayer
+              (position 40.00 40.00)
+              (bounds 180.00 190.00)
+              (drawsContent 1)
+            )
+          )
+        )
+      )
+    )
+  )
+)
+