Make LayoutSVGViewportContainer <-> SVGSVGElement association obvious

third_party/WebKit/Source/core/layout/svg/LayoutSVGViewportContainer.h

Index: third_party/WebKit/Source/core/layout/svg/LayoutSVGViewportContainer.h
diff --git a/third_party/WebKit/Source/core/layout/svg/LayoutSVGViewportContainer.h b/third_party/WebKit/Source/core/layout/svg/LayoutSVGViewportContainer.h
index 389b9b37f018c9faffc20249bc7ec5625b322ec2..713cffdb577a51858526c2d1f8a45e2a0bed4bf7 100644
--- a/third_party/WebKit/Source/core/layout/svg/LayoutSVGViewportContainer.h
+++ b/third_party/WebKit/Source/core/layout/svg/LayoutSVGViewportContainer.h
@@ -27,11 +27,13 @@
 
 namespace blink {
 
+class SVGSVGElement;
+
 // This is used for non-root <svg> elements which are SVGTransformable thus we
 // inherit from LayoutSVGContainer instead of LayoutSVGTransformableContainer.
 class LayoutSVGViewportContainer final : public LayoutSVGContainer {
  public:
-  explicit LayoutSVGViewportContainer(SVGElement*);
+  explicit LayoutSVGViewportContainer(SVGSVGElement*);
   FloatRect viewport() const { return m_viewport; }
 
   bool isLayoutSizeChanged() const { return m_isLayoutSizeChanged; }