cc: Preserve currently scrolling layer during tree activation

BUG=168421


Review URL: https://chromiumcodereview.appspot.com/11826017

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@175729 0039d316-1c4b-4281-b951-d872f2087c98

cc/layer_tree_host_impl.cc

@@ -1025,6 +1025,8 @@ void LayerTreeHostImpl::activatePendingTreeIfNeeded()
 void LayerTreeHostImpl::activatePendingTree()
 {
     CHECK(m_pendingTree);
+
+    m_activeTree->PushPersistedState(m_pendingTree.get());
     m_activeTree.swap(m_pendingTree);
     // TODO(enne): consider recycling this tree to prevent layer churn
     m_pendingTree.reset();

cc/layer_tree_impl.cc

@@ -177,6 +177,12 @@ void LayerTreeImpl::UnregisterLayer(LayerImpl* layer) {
   layer_id_map_.erase(layer->id());
 }
 
+void LayerTreeImpl::PushPersistedState(LayerTreeImpl* pendingTree) {
+  int id = currently_scrolling_layer_ ? currently_scrolling_layer_->id() : 0;
+  pendingTree->set_currently_scrolling_layer(
+      LayerTreeHostCommon::findLayerInSubtree(pendingTree->RootLayer(), id));
+}
+
 const LayerTreeSettings& LayerTreeImpl::settings() const {
   return layer_tree_host_impl_->settings();
 }

cc/layer_tree_impl.h

@@ -129,6 +129,8 @@ class CC_EXPORT LayerTreeImpl {
 
   AnimationRegistrar* animationRegistrar() const;
 
+  void PushPersistedState(LayerTreeImpl* pendingTree);
+
 protected:
   LayerTreeImpl(LayerTreeHostImpl* layer_tree_host_impl);
 
@@ -144,7 +146,7 @@ class CC_EXPORT LayerTreeImpl {
   typedef base::hash_map<int, LayerImpl*> LayerIdMap;
   LayerIdMap layer_id_map_;
 
-  // Persisted state
+  // Persisted state for non-impl-side-painting.
   int scrolling_layer_id_from_previous_tree_;
 
   // List of visible layers for the most recently prepared frame. Used for