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

Source/core/dom/Node.h

Index: Source/core/dom/Node.h
diff --git a/Source/core/dom/Node.h b/Source/core/dom/Node.h
index 58738dcc8f2720a3a7b149df6ea97f8f570ccea4..0f915b46052dc1c99d41a466fc0b9461eaf2c670 100644
--- a/Source/core/dom/Node.h
+++ b/Source/core/dom/Node.h
@@ -39,6 +39,7 @@
 #include "weborigin/KURLHash.h"
 #include "wtf/Forward.h"
 #include "wtf/ListHashSet.h"
+#include "wtf/NonNullPtr.h"
 #include "wtf/text/AtomicString.h"
 
 // This needs to be here because Document.h also depends on it.
@@ -265,7 +266,7 @@ public:
     bool isStyledElement() const { return isHTMLElement() || isSVGElement(); }
 
     bool isDocumentNode() const;
-    bool isTreeScope() const { return treeScope().rootNode() == this; }
+    bool isTreeScope() const { return treeScope()->rootNode() == this; }
     bool isDocumentFragment() const { return getFlag(IsDocumentFragmentFlag); }
     bool isShadowRoot() const { return isDocumentFragment() && isTreeScope(); }
     bool isInsertionPoint() const { return getFlag(IsInsertionPointFlag); }
@@ -480,7 +481,7 @@ public:
         return *documentInternal();
     }
 
-    TreeScope& treeScope() const { return *m_treeScope; }
+    NonNullPtr<TreeScope> treeScope() const { return m_treeScope; }
 
     // Returns true if this node is associated with a document and is in its associated document's
     // node tree, false otherwise.
@@ -805,7 +806,7 @@ protected:
 
     void setHasCustomStyleCallbacks() { setFlag(true, HasCustomStyleCallbacksFlag); }
 
-    Document* documentInternal() const { return treeScope().documentScope(); }
+    Document* documentInternal() const { return treeScope()->documentScope(); }
     void setTreeScope(TreeScope* scope) { m_treeScope = scope; }
 
 private: