Move viewport scrolling logic into separate class

cc/trees/layer_tree_impl.cc

Index: cc/trees/layer_tree_impl.cc
diff --git a/cc/trees/layer_tree_impl.cc b/cc/trees/layer_tree_impl.cc
index 5114fdcecca669bac2dd4d3f5395f9520d2eb1a9..25d3731c5172966d077f9b3475c454088ae6eaa7 100644
--- a/cc/trees/layer_tree_impl.cc
+++ b/cc/trees/layer_tree_impl.cc
@@ -25,6 +25,7 @@
 #include "cc/layers/layer_iterator.h"
 #include "cc/layers/render_surface_impl.h"
 #include "cc/layers/scrollbar_layer_impl_base.h"
+#include "cc/layers/viewport.h"
 #include "cc/resources/ui_resource_request.h"
 #include "cc/trees/layer_tree_host_common.h"
 #include "cc/trees/layer_tree_host_impl.h"
@@ -105,6 +106,7 @@ LayerTreeImpl::LayerTreeImpl(
       top_controls_shrink_blink_size_(false),
       top_controls_height_(0),
       top_controls_shown_ratio_(top_controls_shown_ratio) {
+  viewport_ = Viewport::Create(layer_tree_host_impl);
 }
 
 LayerTreeImpl::~LayerTreeImpl() {
@@ -148,6 +150,8 @@ void LayerTreeImpl::SetRootLayer(scoped_ptr<LayerImpl> layer) {
   inner_viewport_scroll_layer_ = NULL;
   outer_viewport_scroll_layer_ = NULL;
   page_scale_layer_ = NULL;
+  viewport_->SetViewportLayers(inner_viewport_scroll_layer_,
+                               outer_viewport_scroll_layer_);
 
   layer_tree_host_impl_->OnCanDrawStateChangedForTree();
 }
@@ -198,6 +202,8 @@ scoped_ptr<LayerImpl> LayerTreeImpl::DetachLayerTree() {
   outer_viewport_scroll_layer_ = NULL;
   page_scale_layer_ = NULL;
   currently_scrolling_layer_ = NULL;
+  viewport_->SetViewportLayers(inner_viewport_scroll_layer_,
+                               outer_viewport_scroll_layer_);
 
   render_surface_layer_list_.clear();
   set_needs_update_draw_properties();
@@ -513,6 +519,8 @@ void LayerTreeImpl::SetViewportLayersFromIds(
       LayerById(outer_viewport_scroll_layer_id);
   DCHECK(outer_viewport_scroll_layer_ ||
          outer_viewport_scroll_layer_id == Layer::INVALID_ID);
+  viewport_->SetViewportLayers(inner_viewport_scroll_layer_,
+                               outer_viewport_scroll_layer_);
 
   HideInnerViewportScrollbarsIfNearMinimumScale();
 
@@ -535,6 +543,8 @@ void LayerTreeImpl::ClearViewportLayers() {
   page_scale_layer_ = NULL;
   inner_viewport_scroll_layer_ = NULL;
   outer_viewport_scroll_layer_ = NULL;
+  viewport_->SetViewportLayers(inner_viewport_scroll_layer_,
+                               outer_viewport_scroll_layer_);
 }
 
 bool LayerTreeImpl::UpdateDrawProperties(bool update_lcd_text) {