Find root scroll layer at tree activation

cc/layer_tree_host.cc

Index: cc/layer_tree_host.cc
diff --git a/cc/layer_tree_host.cc b/cc/layer_tree_host.cc
index 62f3995601348012d35da8ebd2e4764eec66b495..c237d1a04b6b36bd9d576389e3c6dc94f792ab3b 100644
--- a/cc/layer_tree_host.cc
+++ b/cc/layer_tree_host.cc
@@ -285,7 +285,6 @@ void LayerTreeHost::finishCommitOnImplThread(LayerTreeHostImpl* hostImpl)
         TRACE_EVENT0("cc", "LayerTreeHost::pushProperties");
         TreeSynchronizer::pushProperties(rootLayer(), syncTree->RootLayer());
     }
-    syncTree->FindRootScrollLayer();
 
     m_needsFullTreeSync = false;
 
@@ -298,6 +297,8 @@ void LayerTreeHost::finishCommitOnImplThread(LayerTreeHostImpl* hostImpl)
     syncTree->set_background_color(m_backgroundColor);
     syncTree->set_has_transparent_background(m_hasTransparentBackground);
 
+    syncTree->FindRootScrollLayer();
+
     if (!m_settings.implSidePainting) {
         // If we're not in impl-side painting, the tree is immediately
         // considered active.