Have Node::document() return a reference instead of a pointer

Source/core/dom/MutationObserverInterestGroup.h

Index: Source/core/dom/MutationObserverInterestGroup.h
diff --git a/Source/core/dom/MutationObserverInterestGroup.h b/Source/core/dom/MutationObserverInterestGroup.h
index 2de10700f4b2979fa001642262fa8b5473f6d3cc..4950f31172935cd4f856659f30c47564017d4354 100644
--- a/Source/core/dom/MutationObserverInterestGroup.h
+++ b/Source/core/dom/MutationObserverInterestGroup.h
@@ -44,7 +44,7 @@ class MutationObserverInterestGroup {
 public:
     static PassOwnPtr<MutationObserverInterestGroup> createForChildListMutation(Node* target)
     {
-        if (!target->document()->hasMutationObserversOfType(MutationObserver::ChildList))
+        if (!target->document().hasMutationObserversOfType(MutationObserver::ChildList))
             return nullptr;
 
         MutationRecordDeliveryOptions oldValueFlag = 0;
@@ -53,7 +53,7 @@ public:
 
     static PassOwnPtr<MutationObserverInterestGroup> createForCharacterDataMutation(Node* target)
     {
-        if (!target->document()->hasMutationObserversOfType(MutationObserver::CharacterData))
+        if (!target->document().hasMutationObserversOfType(MutationObserver::CharacterData))
             return nullptr;
 
         return createIfNeeded(target, MutationObserver::CharacterData, MutationObserver::CharacterDataOldValue);
@@ -61,7 +61,7 @@ public:
 
     static PassOwnPtr<MutationObserverInterestGroup> createForAttributesMutation(Node* target, const QualifiedName& attributeName)
     {
-        if (!target->document()->hasMutationObserversOfType(MutationObserver::Attributes))
+        if (!target->document().hasMutationObserversOfType(MutationObserver::Attributes))
             return nullptr;
 
         return createIfNeeded(target, MutationObserver::Attributes, MutationObserver::AttributeOldValue, &attributeName);