Simplify some shadow crossing logic in SelectorQuery.cpp

ElementTraversal::firstWithin is the same as doing
::next(rootNode, &rootNode) we can just do that here. We can also use
isOpenOrV0() to remove some verbose checks, and finally we don't need to
check isShadowHost (which contains a bunch of duplciate checks) and can
just null check ::shadow() which is equivalent.

BUG=703900

Review-Url: https://codereview.chromium.org/2782483004
Cr-Commit-Position: refs/heads/master@{#460544}

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

@@ -363,27 +363,20 @@ void SelectorQuery::executeSlow(
 // firstWithinTraversingShadowTree, nextTraversingShadowTree to the best place,
 // e.g. NodeTraversal.
 static ShadowRoot* authorShadowRootOf(const ContainerNode& node) {
-  if (!node.isElementNode() || !isShadowHost(&node))
+  if (!node.isElementNode())
     return nullptr;
-
   ElementShadow* shadow = toElement(node).shadow();
-  DCHECK(shadow);
+  if (!shadow)
+    return nullptr;
+
   for (ShadowRoot* shadowRoot = &shadow->oldestShadowRoot(); shadowRoot;
        shadowRoot = shadowRoot->youngerShadowRoot()) {
-    if (shadowRoot->type() == ShadowRootType::V0 ||
-        shadowRoot->type() == ShadowRootType::Open)
+    if (shadowRoot->isOpenOrV0())
       return shadowRoot;
   }
   return nullptr;
 }
 
-static ContainerNode* firstWithinTraversingShadowTree(
-    const ContainerNode& rootNode) {
-  if (ShadowRoot* shadowRoot = authorShadowRootOf(rootNode))
-    return shadowRoot;
-  return ElementTraversal::firstWithin(rootNode);
-}
-
 static ContainerNode* nextTraversingShadowTree(const ContainerNode& node,
                                                const ContainerNode* rootNode) {
   if (ShadowRoot* shadowRoot = authorShadowRootOf(node))
@@ -401,9 +394,7 @@ static ContainerNode* nextTraversingShadowTree(const ContainerNode& node,
     if (shadowRoot == rootNode)
       return nullptr;
     if (ShadowRoot* youngerShadowRoot = shadowRoot->youngerShadowRoot()) {
-      // Should not obtain any elements in closed or user-agent shadow root.
-      DCHECK(youngerShadowRoot->type() == ShadowRootType::V0 ||
-             youngerShadowRoot->type() == ShadowRootType::Open);
+      DCHECK(youngerShadowRoot->isOpenOrV0());
       return youngerShadowRoot;
     }
 
@@ -416,8 +407,8 @@ template <typename SelectorQueryTrait>
 void SelectorQuery::executeSlowTraversingShadowTree(
     ContainerNode& rootNode,
     typename SelectorQueryTrait::OutputType& output) const {
-  for (ContainerNode* node = firstWithinTraversingShadowTree(rootNode); node;
-       node = nextTraversingShadowTree(*node, &rootNode)) {
+  for (ContainerNode* node = nextTraversingShadowTree(rootNode, &rootNode);
+       node; node = nextTraversingShadowTree(*node, &rootNode)) {
     if (!node->isElementNode())
       continue;
     Element* element = toElement(node);