Add a number of tests for layer squashing

LayoutTests/compositing/squashing/squash-onto-nephew-expected.txt

+(GraphicsLayer
+  (bounds 800.00 600.00)
+  (children 1
+    (GraphicsLayer
+      (bounds 800.00 600.00)
+      (contentsOpaque 1)
+      (drawsContent 1)
+      (children 1
+        (GraphicsLayer
+          (position 25.00 25.00)
+          (bounds 300.00 300.00)
+          (contentsOpaque 1)
+          (backgroundColor #808080)
+          (children 1
+            (GraphicsLayer
+              (bounds 300.00 300.00)
+              (children 1
+                (GraphicsLayer
+                  (children 2
+                    (GraphicsLayer
+                      (position 100.00 100.00)
+                      (bounds 100.00 100.00)
+                      (contentsOpaque 1)
+                    )
+                    (GraphicsLayer
+                      (bounds 300.00 300.00)

[shawnsingh, 2014/01/28 22:56:25]

I tested this on canary... unfortunately squashing does not do what I expect to
be correct.  Maybe we have to remove this test from this patch and create a bug
for it.

[abarth-chromium, 2014/01/28 23:01:43]

What behavior do you think is correct?  We can land the test with the correct
expectation and file a bug about not matching it.

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