DOM: Stop inlining Node::parentNode().

This is a preparation to add a UseCounter in Node::parentNode().

Bug: 859391
Change-Id: I056f60207e000b6f9b1acd6e412444f96a8daee2
Reviewed-on: https://chromium-review.googlesource.com/1121944Reviewed-by: Hayato Ito <hayato@chromium.org>
Commit-Queue: Kent Tamura <tkent@chromium.org>
Cr-Commit-Position: refs/heads/master@{#571831}

third_party/blink/renderer/core/dom/node.cc

@@ -357,6 +357,10 @@ void Node::setNodeValue(const String&) {
   // By default, setting nodeValue has no effect.
 }
 
+ContainerNode* Node::parentNode() const {
+  return IsShadowRoot() ? nullptr : ParentOrShadowHostNode();
+}
+
 NodeList* Node::childNodes() {
   ThreadState::MainThreadGCForbiddenScope gc_forbidden;
   if (IsContainerNode())

third_party/blink/renderer/core/dom/node.h

@@ -1030,10 +1030,6 @@ inline ContainerNode* Node::ParentOrShadowHostNode() const {
   return reinterpret_cast<ContainerNode*>(parent_or_shadow_host_node_.Get());
 }
 
-inline ContainerNode* Node::parentNode() const {
-  return IsShadowRoot() ? nullptr : ParentOrShadowHostNode();
-}
-
 inline void Node::LazyReattachIfAttached() {
   if (NeedsAttach())
     return;