[cc] Fix crash when adding a child layer to its own parent.

cc/layer_unittest.cc

Index: cc/layer_unittest.cc
diff --git a/cc/layer_unittest.cc b/cc/layer_unittest.cc
index 2bbbef4db906dea6fde27092e775716adcde02b4..92b150efa5cfc009f0032d0ec606e350a45cdf0f 100644
--- a/cc/layer_unittest.cc
+++ b/cc/layer_unittest.cc
@@ -164,6 +164,24 @@ TEST_F(LayerTest, addAndRemoveChild)
     EXECUTE_AND_VERIFY_SET_NEEDS_COMMIT_BEHAVIOR(AtLeast(1), child->removeFromParent());
 }
 
+TEST_F(LayerTest, addSameChildTwice)
+{
+    scoped_refptr<Layer> parent = Layer::create();
+    scoped_refptr<Layer> child = Layer::create();
+
+    m_layerTreeHost->setRootLayer(parent);
+
+    ASSERT_EQ(0u, parent->children().size());
+
+    parent->addChild(child);
+    ASSERT_EQ(1u, parent->children().size());
+    EXPECT_EQ(parent.get(), child->parent());
+
+    parent->addChild(child);
+    ASSERT_EQ(1u, parent->children().size());
+    EXPECT_EQ(parent.get(), child->parent());
+}
+
 TEST_F(LayerTest, insertChild)
 {
     scoped_refptr<Layer> parent = Layer::create();