[cc] Make LayerImpls point at LayerTreeImpl instead of LTHI

cc/layer_tree_host_impl.cc

Index: cc/layer_tree_host_impl.cc
diff --git a/cc/layer_tree_host_impl.cc b/cc/layer_tree_host_impl.cc
index f6b753915070bb426cf9fcb728db7a9f9d36ec29..034ef0815e638802263b18c34f47d88f54ad2219 100644
--- a/cc/layer_tree_host_impl.cc
+++ b/cc/layer_tree_host_impl.cc
@@ -20,6 +20,7 @@
 #include "cc/layer_iterator.h"
 #include "cc/layer_tree_host.h"
 #include "cc/layer_tree_host_common.h"
+#include "cc/layer_tree_impl.h"
 #include "cc/math_util.h"
 #include "cc/overdraw_metrics.h"
 #include "cc/page_scale_animation.h"
@@ -955,6 +956,21 @@ void LayerTreeHostImpl::readback(void* pixels, const gfx::Rect& rect)
     m_renderer->getFramebufferPixels(pixels, rect);
 }
 
+LayerImpl* LayerTreeHostImpl::rootLayer() const
+{
+  return m_activeTree->RootLayer();
+}
+
+LayerImpl* LayerTreeHostImpl::rootScrollLayer() const
+{
+  return m_activeTree->root_scroll_layer();
+}
+
+LayerImpl* LayerTreeHostImpl::currentlyScrollingLayer() const
+{
+  return m_activeTree->currently_scrolling_layer();
+}
+
 // Content layers can be either directly scrollable or contained in an outer
 // scrolling layer which applies the scroll transform. Given a content layer,
 // this function returns the associated scroll layer if any.