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

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 f959d8ec41e4a855031c51d2ab5fa71fcccecec9..7a7c9351f304fb513a6b3be7ce95e85b5538f22a 100644
--- a/cc/trees/layer_tree_impl.cc
+++ b/cc/trees/layer_tree_impl.cc
@@ -111,6 +111,11 @@ void LayerTreeImpl::PushPropertiesTo(LayerTreeImpl* target_tree) {
       target_tree->page_scale_delta() / target_tree->sent_page_scale_delta());
   target_tree->set_sent_page_scale_delta(1);
 
+  if (settings().use_pinch_virtual_viewport)
+    target_tree->SetPinchViewportLayerIds(page_scale_layer_->id(),
+                                          inner_viewport_scroll_layer_->id(),
+                                          outer_viewport_scroll_layer_->id());
+
   // This should match the property synchronization in
   // LayerTreeHost::finishCommitOnImplThread().
   target_tree->set_source_frame_number(source_frame_number());
@@ -222,6 +227,17 @@ void LayerTreeImpl::UpdateMaxScrollOffset() {
   root_scroll_layer_->SetMaxScrollOffset(gfx::ToFlooredVector2d(max_scroll));
 }
 
+void LayerTreeImpl::SetPinchViewportLayerIds(
+    int page_scale_layer_id,
+    int inner_viewport_scroll_layer_id,
+    int outer_viewport_scroll_layer_id) {
+  page_scale_layer_ = FindPendingTreeLayerById(page_scale_layer_id);
+  inner_viewport_scroll_layer_ =
+      FindPendingTreeLayerById(inner_viewport_scroll_layer_id);
+  outer_viewport_scroll_layer_ =
+      FindPendingTreeLayerById(outer_viewport_scroll_layer_id);
+}
+
 void LayerTreeImpl::UpdateSolidColorScrollbars() {
   DCHECK(settings().solid_color_scrollbars);
 
@@ -280,13 +296,14 @@ void LayerTreeImpl::UpdateDrawProperties() {
                  "LayerTreeImpl::UpdateDrawProperties",
                  "IsActive",
                  IsActiveTree());
+
     LayerTreeHostCommon::CalculateDrawProperties(
         root_layer(),
         layer_tree_host_impl_->DeviceViewport().size(),
         layer_tree_host_impl_->DeviceTransform(),
         device_scale_factor(),
         total_page_scale_factor(),
-        root_scroll_layer_,
+        page_scale_layer_ ? page_scale_layer_ : root_scroll_layer_,
         MaxTextureSize(),
         settings().can_use_lcd_text,
         settings().layer_transforms_should_scale_layer_contents,