Reduce number of header includes in SVG

Source/core/svg/SVGElementInstanceList.h

Index: Source/core/svg/SVGElementInstanceList.h
diff --git a/Source/core/svg/SVGElementInstanceList.h b/Source/core/svg/SVGElementInstanceList.h
index 4b54fd12c529630cbcf55ba8ab7ad28051c9faf1..f0fe1eba8efdd7db20aeff1634af317b7eb9c0ed 100644
--- a/Source/core/svg/SVGElementInstanceList.h
+++ b/Source/core/svg/SVGElementInstanceList.h
@@ -21,11 +21,12 @@
 #define SVGElementInstanceList_h
 
 #include "bindings/v8/ScriptWrappable.h"
-#include "core/svg/SVGElementInstance.h"
 #include "wtf/RefCounted.h"
 
 namespace WebCore {
 
+class SVGElementInstance;
+
 class SVGElementInstanceList : public RefCounted<SVGElementInstanceList>, public ScriptWrappable {
 public:
     static PassRefPtr<SVGElementInstanceList> create(PassRefPtr<SVGElementInstance> root) { return adoptRef(new SVGElementInstanceList(root)); }