Code cleanup: Reorder code in ContainerNode::InsertBefore().

so that it matches to the DOM specification more.
This CL has no behavior changes.

Bug: 730412
Change-Id: I8881f2286e098238e255553bea8d2e1ab8b13ca4
Reviewed-on: https://chromium-review.googlesource.com/530924Reviewed-by: Hayato Ito <hayato@chromium.org>
Commit-Queue: Kent Tamura <tkent@chromium.org>
Cr-Commit-Position: refs/heads/master@{#478563}

third_party/WebKit/Source/core/dom/ContainerNode.cpp

@@ -356,10 +356,15 @@ Node* ContainerNode::InsertBefore(Node* new_child,
   DCHECK(new_child);
   // https://dom.spec.whatwg.org/#concept-node-pre-insert
 
-  // insertBefore(node, 0) is equivalent to appendChild(node)
+  // insertBefore(node, null) is equivalent to appendChild(node)
   if (!ref_child)
     return AppendChild(new_child, exception_state);
 
+  // 1. Ensure pre-insertion validity of node into parent before child.
+  if (!EnsurePreInsertionValidity(*new_child, ref_child, nullptr,
+                                  exception_state))
+    return new_child;
+
   // 2. Let reference child be child.
   // 3. If reference child is node, set it to node’s next sibling.
   if (ref_child == new_child) {
@@ -368,11 +373,7 @@ Node* ContainerNode::InsertBefore(Node* new_child,
       return AppendChild(new_child, exception_state);
   }
 
-  // 1. Ensure pre-insertion validity of node into parent before child.
-  if (!EnsurePreInsertionValidity(*new_child, ref_child, nullptr,
-                                  exception_state))
-    return new_child;
-
+  // 4. Adopt node into parent’s node document.
   NodeVector targets;
   DOMTreeMutationDetector detector(*new_child, *this);
   if (!CollectChildrenAndRemoveFromOldParent(*new_child, targets,
@@ -384,6 +385,7 @@ Node* ContainerNode::InsertBefore(Node* new_child,
       return new_child;
   }
 
+  // 5. Insert node into parent before reference child.
   NodeVector post_insertion_notification_targets;
   {
     ChildListMutationScope mutation(*this);