Make RootScroller set the outer viewport scroll layer in the compositor

cc/trees/layer_tree_host_impl.cc

Index: cc/trees/layer_tree_host_impl.cc
diff --git a/cc/trees/layer_tree_host_impl.cc b/cc/trees/layer_tree_host_impl.cc
index 53020f0522d00a13ce9c9cc55e55929cfe1c0380..e4d77f5b15b9d5768389de7dfc37e6f3983a94b0 100644
--- a/cc/trees/layer_tree_host_impl.cc
+++ b/cc/trees/layer_tree_host_impl.cc
@@ -82,6 +82,7 @@
 #include "cc/trees/layer_tree_host.h"
 #include "cc/trees/layer_tree_host_common.h"
 #include "cc/trees/layer_tree_impl.h"
+#include "cc/trees/root_scroller_controller.h"
 #include "cc/trees/single_thread_proxy.h"
 #include "cc/trees/tree_synchronizer.h"
 #include "gpu/GLES2/gl2extchromium.h"
@@ -266,6 +267,7 @@ LayerTreeHostImpl::LayerTreeHostImpl(
       TopControlsManager::Create(this,
                                  settings.top_controls_show_threshold,
                                  settings.top_controls_hide_threshold);
+  root_scroller_controller_ = RootScrollerController::Create();
 }
 
 LayerTreeHostImpl::~LayerTreeHostImpl() {
@@ -2032,6 +2034,9 @@ void LayerTreeHostImpl::ActivateSyncTree() {
   active_tree_->UpdatePropertyTreesForBoundsDelta();
   UpdateViewportContainerSizes();
 
+  root_scroller_controller_->SetRootScrollerLayer(
+      active_tree_->RootScrollerLayer());
+
   active_tree_->DidBecomeActive();
   client_->RenewTreePriority();
   // If we have any picture layers, then by activating we also modified tile
@@ -3049,10 +3054,18 @@ void LayerTreeHostImpl::DistributeScrollDelta(ScrollState* scroll_state) {
       // Skip the outer viewport scroll layer so that we try to scroll the
       // viewport only once. i.e. The inner viewport layer represents the
       // viewport.
-      if (!scroll_node->data.scrollable ||
-          scroll_node->data.is_outer_viewport_scroll_layer)
-        continue;
-      current_scroll_chain.push_front(scroll_node);
+      if (scroll_node->data.scrollable &&
+          !scroll_node->data.is_outer_viewport_scroll_layer)
+        current_scroll_chain.push_front(scroll_node);
+
+      // TODO(bokan): We should modify the scroll tree to be built to take
+      // account of the root scroller.
+      // If we're reached the root scroller, don't keep chaining.
+      LayerImpl* root_scroller_layer =
+          root_scroller_controller()->GetRootScrollerLayer();
+      if (root_scroller_layer &&
+          scroll_node->owner_id == root_scroller_layer->id())
+        break;
     }
   }
   scroll_state->set_scroll_chain_and_layer_tree(current_scroll_chain,