Make ElementShadow::oldestShadowRoot() return a reference

ElementShadow::m_shadowRoot is always non-nullptr here.

BUG=624724

Review-Url: https://codereview.chromium.org/2322223002
Cr-Commit-Position: refs/heads/master@{#417559}

third_party/WebKit/Source/core/css/resolver/StyleResolver.cpp

@@ -421,7 +421,7 @@ static void matchHostRules(const Element& element, ElementRuleCollector& collect
     if (!shadow)
         return;
 
-    for (ShadowRoot* shadowRoot = shadow->oldestShadowRoot(); shadowRoot; shadowRoot = shadowRoot->youngerShadowRoot()) {
+    for (ShadowRoot* shadowRoot = &shadow->oldestShadowRoot(); shadowRoot; shadowRoot = shadowRoot->youngerShadowRoot()) {
         if (!shadowRoot->numberOfStyles())
             continue;
         if (ScopedStyleResolver* resolver = shadowRoot->scopedStyleResolver()) {

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

@@ -2119,10 +2119,9 @@ ShadowRoot* Element::authorShadowRoot() const
 ShadowRoot* Element::userAgentShadowRoot() const
 {
     if (ElementShadow* elementShadow = shadow()) {
-        if (ShadowRoot* root = elementShadow->oldestShadowRoot()) {
-            DCHECK(root->type() == ShadowRootType::UserAgent);
-            return root;
-        }
+        ShadowRoot& root = elementShadow->oldestShadowRoot();
+        DCHECK(root.type() == ShadowRootType::UserAgent);
+        return &root;
     }
 
     return nullptr;

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

@@ -580,7 +580,7 @@ inline static ShadowRoot* oldestShadowRootFor(const Node* node)
     if (!node->isElementNode())
         return nullptr;
     if (ElementShadow* shadow = toElement(node)->shadow())
-        return shadow->oldestShadowRoot();
+        return &shadow->oldestShadowRoot();
     return nullptr;
 }
 #endif

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

@@ -393,7 +393,7 @@ static ShadowRoot* authorShadowRootOf(const ContainerNode& node)
 
     ElementShadow* shadow = toElement(node).shadow();
     DCHECK(shadow);
-    for (ShadowRoot* shadowRoot = shadow->oldestShadowRoot(); shadowRoot; shadowRoot = shadowRoot->youngerShadowRoot()) {
+    for (ShadowRoot* shadowRoot = &shadow->oldestShadowRoot(); shadowRoot; shadowRoot = shadowRoot->youngerShadowRoot()) {
         if (shadowRoot->type() == ShadowRootType::V0 || shadowRoot->type() == ShadowRootType::Open)
             return shadowRoot;
     }

third_party/WebKit/Source/core/dom/shadow/ElementShadow.cpp

@@ -332,7 +332,7 @@ const SelectRuleFeatureSet& ElementShadow::ensureSelectFeatureSet()
         return m_selectFeatures;
 
     m_selectFeatures.clear();
-    for (ShadowRoot* root = oldestShadowRoot(); root; root = root->youngerShadowRoot())
+    for (ShadowRoot* root = &oldestShadowRoot(); root; root = root->youngerShadowRoot())
         collectSelectFeatureSetFrom(*root);
     m_needsSelectFeatureSet = false;
     return m_selectFeatures;

third_party/WebKit/Source/core/dom/shadow/ElementShadow.h

@@ -45,7 +45,7 @@ public:
 
     Element& host() const;
     ShadowRoot& youngestShadowRoot() const;
-    ShadowRoot* oldestShadowRoot() const { return m_shadowRoot; }
+    ShadowRoot& oldestShadowRoot() const { DCHECK(m_shadowRoot); return *m_shadowRoot; }
     ElementShadow* containingShadow() const;
 
     ShadowRoot& addShadowRoot(Element& shadowHost, ShadowRootType);

third_party/WebKit/Source/core/testing/Internals.cpp

@@ -692,7 +692,7 @@ ShadowRoot* Internals::oldestShadowRoot(Element* host)
 {
     ASSERT(host);
     if (ElementShadow* shadow = host->shadow())
-        return shadow->oldestShadowRoot();
+        return &shadow->oldestShadowRoot();
     return 0;
 }