Add viewport scrollbar class to support overlay scrollbars for pinch zoom virtual viewport.

webkit/renderer/compositor_bindings/web_compositor_support_impl.h

Index: webkit/renderer/compositor_bindings/web_compositor_support_impl.h
diff --git a/webkit/renderer/compositor_bindings/web_compositor_support_impl.h b/webkit/renderer/compositor_bindings/web_compositor_support_impl.h
index 9d7f859e58488a62776e75bc4058e5d2914a4311..f4ab45f0e96fe6aa4dc953ad99a8dfee1b80a415 100644
--- a/webkit/renderer/compositor_bindings/web_compositor_support_impl.h
+++ b/webkit/renderer/compositor_bindings/web_compositor_support_impl.h
@@ -9,6 +9,7 @@
 #include "base/message_loop/message_loop_proxy.h"
 #include "third_party/WebKit/public/platform/WebCompositorSupport.h"
 #include "third_party/WebKit/public/platform/WebLayer.h"
+#include "third_party/WebKit/public/platform/WebScrollbar.h"
 #include "third_party/WebKit/public/platform/WebTransformOperations.h"
 
 namespace WebKit {
@@ -36,6 +37,10 @@ class WebCompositorSupportImpl : public WebKit::WebCompositorSupport {
       WebKit::WebScrollbar* scrollbar,
       WebKit::WebScrollbarThemePainter painter,
       WebKit::WebScrollbarThemeGeometry*);
+  virtual WebKit::WebScrollbarLayer* createScrollbarLayer(

[enne (OOO), 2013/06/14 20:56:59]

I had a conversation with jamesr who suggested that cc create these scrollbar
layers itself based on layer ids passed into LayerTreeHost.  That way we don't
have to plumb these special scrollbar types all the way through compositor
bindings.  That seems like a better approach to me too.

[wjmaclean, 2013/06/17 13:44:28]

Yes, I will remove the compositor bindings parts, and put up a new CL.

+      WebKit::WebLayer* scroll_layer,
+      WebKit::WebScrollbar::Orientation orientation,
+      size_t thickness);
   virtual WebKit::WebAnimation* createAnimation(
       const WebKit::WebAnimationCurve& curve,
       WebKit::WebAnimation::TargetProperty target,