SVG: Move/rename reference management to SVGElement

Source/core/svg/SVGElement.h

Index: Source/core/svg/SVGElement.h
diff --git a/Source/core/svg/SVGElement.h b/Source/core/svg/SVGElement.h
index 30b43959e52e3782a7946c08374f6833896f8edf..9a63a22e644f8cb3382226cc40a44b7bc36c5cbd 100644
--- a/Source/core/svg/SVGElement.h
+++ b/Source/core/svg/SVGElement.h
@@ -40,12 +40,15 @@ class SVGAnimatedPropertyBase;
 class SubtreeLayoutScope;
 class SVGCursorElement;
 class SVGDocumentExtensions;
+class SVGElement;
 class SVGElementRareData;
 class SVGFitToViewBox;
 class SVGSVGElement;
 
 void mapAttributeToCSSProperty(HashMap<StringImpl*, CSSPropertyID>* propertyNameToIdMap, const QualifiedName& attrName);
 
+typedef WillBeHeapHashSet<RawPtrWillBeMember<SVGElement> > SVGElementSet;
+
 class SVGElement : public Element {
 public:
     virtual ~SVGElement();
@@ -159,6 +162,12 @@ public:
 
     bool inUseShadowTree() const;
 
+    SVGElementSet* setOfIncomingReferences() const;
+    void addReferenceTo(SVGElement*);
+    void rebuildAllIncomingReferences();
+    void removeAllIncomingReferences();
+    void removeAllOutgoingReferences();
+
     class InvalidationGuard {
         STACK_ALLOCATED();
         WTF_MAKE_NONCOPYABLE(InvalidationGuard);
@@ -207,8 +216,6 @@ protected:
 
     virtual bool selfHasRelativeLengths() const { return false; }
 
-    // FIXME: This "friend" is temporary to keep the current |SVGDocumentExtensions::*ReferencesFor*| methods. This will be removed after they are moved to SVGElement.
-    friend class SVGDocumentExtensions;
     SVGElementRareData* ensureSVGRareData();
     inline bool hasSVGRareData() const { return m_SVGRareData; }
     inline SVGElementRareData* svgRareData() const