WIP (Introduce WTF::NonNullPtr<T>.)

Source/core/accessibility/AccessibilityNodeObject.cpp

Index: Source/core/accessibility/AccessibilityNodeObject.cpp
diff --git a/Source/core/accessibility/AccessibilityNodeObject.cpp b/Source/core/accessibility/AccessibilityNodeObject.cpp
index 7e7bc64e6132e24a03c3843f751ff7c03d6d7ffe..668ec8c7f11d029671d0d18bbe9adef5a6a3225d 100644
--- a/Source/core/accessibility/AccessibilityNodeObject.cpp
+++ b/Source/core/accessibility/AccessibilityNodeObject.cpp
@@ -259,7 +259,7 @@ void AccessibilityNodeObject::elementsFromAttribute(Vector<Element*>& elements,
     if (!node || !node->isElementNode())
         return;
 
-    TreeScope& scope = node->treeScope();
+    NonNullPtr<TreeScope> scope = node->treeScope();
 
     String idList = getAttribute(attribute).string();
     if (idList.isEmpty())
@@ -272,7 +272,7 @@ void AccessibilityNodeObject::elementsFromAttribute(Vector<Element*>& elements,
     unsigned size = idVector.size();
     for (unsigned i = 0; i < size; ++i) {
         AtomicString idName(idVector[i]);
-        Element* idElement = scope.getElementById(idName);
+        Element* idElement = scope->getElementById(idName);
         if (idElement)
             elements.append(idElement);
     }
@@ -341,7 +341,7 @@ HTMLLabelElement* AccessibilityNodeObject::labelForElement(Element* element) con
 
     const AtomicString& id = element->getIdAttribute();
     if (!id.isEmpty()) {
-        if (HTMLLabelElement* label = element->treeScope().labelElementForId(id))
+        if (HTMLLabelElement* label = element->treeScope()->labelElementForId(id))
             return label;
     }