Find root scroll layer at tree activation

cc/layer_tree_host_impl.h

Index: cc/layer_tree_host_impl.h
diff --git a/cc/layer_tree_host_impl.h b/cc/layer_tree_host_impl.h
index 3d92dc4fb19f667604cf73d4c85fb33b1146d39e..7eed7e7442b00e258e23c8bb00b18e853112de2b 100644
--- a/cc/layer_tree_host_impl.h
+++ b/cc/layer_tree_host_impl.h
@@ -82,9 +82,9 @@ public:
     virtual bool haveTouchEventHandlersAt(const gfx::Point&) OVERRIDE;
 
     // TopControlsManagerClient implementation.
-    virtual LayerTreeImpl* activeTree() OVERRIDE;
     virtual void setNeedsUpdateDrawProperties() OVERRIDE;
     virtual void setNeedsRedraw() OVERRIDE;
+    virtual LayerImpl* rootScrollLayer() const OVERRIDE;
 
     struct CC_EXPORT FrameData : public RenderPassSink {
         FrameData();
@@ -159,6 +159,7 @@ public:
 
     void readback(void* pixels, const gfx::Rect&);
 
+    LayerTreeImpl* activeTree() { return m_activeTree.get(); }
     const LayerTreeImpl* activeTree() const { return m_activeTree.get(); }
     LayerTreeImpl* pendingTree() { return m_pendingTree.get(); }
     const LayerTreeImpl* pendingTree() const { return m_pendingTree.get(); }
@@ -168,7 +169,6 @@ public:
 
     // Shortcuts to layers on the active tree.
     LayerImpl* rootLayer() const;
-    LayerImpl* rootScrollLayer() const;
     LayerImpl* currentlyScrollingLayer() const;
 
     bool visible() const { return m_visible; }