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

content/renderer/gpu/render_widget_compositor.cc

Index: content/renderer/gpu/render_widget_compositor.cc
diff --git a/content/renderer/gpu/render_widget_compositor.cc b/content/renderer/gpu/render_widget_compositor.cc
index 55c33858686b5730a68d5dab7bbe9f0dc6f54760..2ffb368b4cc1ac61c4d32eb9b50f15abaae52b05 100644
--- a/content/renderer/gpu/render_widget_compositor.cc
+++ b/content/renderer/gpu/render_widget_compositor.cc
@@ -472,6 +472,37 @@ void RenderWidgetCompositor::registerForAnimations(WebKit::WebLayer* layer) {
       layer_tree_host_->animation_registrar());
 }
 
+void RenderWidgetCompositor::registerPinchViewportLayers(
+    const WebKit::WebLayer* innerViewportClipLayer,
+    const WebKit::WebLayer* pageScaleLayerLayer,
+    const WebKit::WebLayer* innerViewportScrollLayer,
+    const WebKit::WebLayer* outerViewportScrollLayer,
+    const WebKit::WebLayer* innerViewportHorizontalScrollbarLayer,
+    const WebKit::WebLayer* innerViewportVerticalScrollbarLayer) {
+  layer_tree_host_->RegisterPinchViewportLayers(
+      static_cast<const webkit::WebLayerImpl*>(innerViewportClipLayer)
+          ->layer(),
+      static_cast<const webkit::WebLayerImpl*>(pageScaleLayerLayer)->layer(),
+      static_cast<const webkit::WebLayerImpl*>(innerViewportScrollLayer)
+          ->layer(),
+      static_cast<const webkit::WebLayerImpl*>(outerViewportScrollLayer)
+          ->layer(),
+      static_cast<const webkit::WebLayerImpl*>(
+          innerViewportHorizontalScrollbarLayer)->layer(),
+      static_cast<const webkit::WebLayerImpl*>(
+          innerViewportVerticalScrollbarLayer)->layer());
+}
+
+void RenderWidgetCompositor::clearPinchViewportLayers() {
+  layer_tree_host_->RegisterPinchViewportLayers(
+      scoped_refptr<cc::Layer>(),
+      scoped_refptr<cc::Layer>(),
+      scoped_refptr<cc::Layer>(),
+      scoped_refptr<cc::Layer>(),
+      scoped_refptr<cc::Layer>(),
+      scoped_refptr<cc::Layer>());
+}
+
 bool RenderWidgetCompositor::compositeAndReadback(
     void *pixels, const WebRect& rect_in_device_viewport) {
   return layer_tree_host_->CompositeAndReadback(pixels,