Add a number of tests for layer squashing

LayoutTests/compositing/squashing/squash-clipped-expected.txt

+(GraphicsLayer
+  (bounds 800.00 600.00)
+  (children 1
+    (GraphicsLayer
+      (bounds 800.00 600.00)
+      (contentsOpaque 1)
+      (drawsContent 1)
+      (children 1
+        (GraphicsLayer
+          (position 8.00 8.00)
+          (bounds 400.00 400.00)
+          (contentsOpaque 1)
+          (drawsContent 1)
+          (backgroundColor #808080)
+          (children 1
+            (GraphicsLayer
+              (bounds 400.00 400.00)
+              (children 1
+                (GraphicsLayer
+                  (children 2
+                    (GraphicsLayer
+                      (position 100.00 100.00)
+                      (bounds 100.00 100.00)
+                      (contentsOpaque 1)
+                    )
+                    (GraphicsLayer
+                      (position 180.00 180.00)
+                      (bounds 260.00 260.00)

[shawnsingh, 2014/01/27 21:35:27]

It makes sense that these bounds are 260x260, but that means there's no
indication in the test results that it was properly clipped.  If nothing else,
maybe we make this a pixel test to capture the clipping correctness.  Or perhaps
there's a better way that I'm not aware of...

+                      (drawsContent 1)
+                    )
+                  )
+                )
+              )
+            )
+          )
+        )
+      )
+    )
+  )
+)
+