Insert pinch zoom virtual viewport layers to graphics layer tree.

public/platform/WebCompositorSupport.h

Index: public/platform/WebCompositorSupport.h
diff --git a/public/platform/WebCompositorSupport.h b/public/platform/WebCompositorSupport.h
index f3b8411d005bc963f603a48e51a85e1e6c7e52df..b5cc2a0e9fd52f965b8a4441ae95ad15c3d14fe3 100644
--- a/public/platform/WebCompositorSupport.h
+++ b/public/platform/WebCompositorSupport.h
@@ -29,6 +29,7 @@
 #include "WebAnimation.h"
 #include "WebCommon.h"
 #include "WebLayerTreeView.h"
+#include "WebScrollbar.h"
 #include "WebScrollbarThemePainter.h"
 
 namespace WebKit {
@@ -43,7 +44,6 @@ class WebFloatAnimationCurve;
 class WebGraphicsContext3D;
 class WebImageLayer;
 class WebLayer;
-class WebScrollbar;
 class WebScrollbarLayer;
 class WebScrollbarThemeGeometry;
 class WebSolidColorLayer;
@@ -75,6 +75,8 @@ public:
 
     virtual WebScrollbarLayer* createScrollbarLayer(WebScrollbar*, WebScrollbarThemePainter, WebScrollbarThemeGeometry*) { return 0; }
 
+    virtual WebScrollbarLayer* createScrollbarLayer(WebLayer* scrollLayer, WebScrollbar::Orientation, size_t thickness) { return 0; }
+
 
     // Animation ----------------------------------------------------