Make RootScroller set the outer viewport scroll layer in the compositor

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 c73ca064a9d6fcc57e93ca15291a46c23b664ace..30b81c678e4eebeb5b6f2f725c4ad9984382f1c6 100644
--- a/content/renderer/gpu/render_widget_compositor.cc
+++ b/content/renderer/gpu/render_widget_compositor.cc
@@ -782,8 +782,6 @@ void RenderWidgetCompositor::registerViewportLayers(
       // TODO(bokan): This check can probably be removed now, but it looks
       // like overscroll elasticity may still be NULL until VisualViewport
       // registers its layers.
-      // The scroll elasticity layer will only exist when using pinch virtual
-      // viewports.
       overscrollElasticityLayer
           ? static_cast<const cc_blink::WebLayerImpl*>(
                 overscrollElasticityLayer)->layer()
@@ -792,10 +790,8 @@ void RenderWidgetCompositor::registerViewportLayers(
       static_cast<const cc_blink::WebLayerImpl*>(innerViewportScrollLayer)
           ->layer(),
       // TODO(bokan): This check can probably be removed now, but it looks
-      // like overscroll elasticity may still be NULL until VisualViewport
+      // like outer viewport may still be NULL until VisualViewport
       // registers its layers.
-      // The outer viewport layer will only exist when using pinch virtual
-      // viewports.
       outerViewportScrollLayer
           ? static_cast<const cc_blink::WebLayerImpl*>(outerViewportScrollLayer)
                 ->layer()
@@ -808,6 +804,13 @@ void RenderWidgetCompositor::clearViewportLayers() {
       scoped_refptr<cc::Layer>(), scoped_refptr<cc::Layer>());
 }
 
+void RenderWidgetCompositor::setRootScrollerLayer(
+    const blink::WebLayer* rootScrollerLayer) {
+  layer_tree_host_->SetRootScrollerLayer(rootScrollerLayer
+      ? static_cast<const cc_blink::WebLayerImpl*>(rootScrollerLayer)->layer()
+      : NULL);
+}
+
 void RenderWidgetCompositor::registerSelection(
     const blink::WebSelection& selection) {
   layer_tree_host_->RegisterSelection(ConvertWebSelection(selection));