wip: not for commit (update twin relationship)

cc/layers/picture_layer_impl.cc

Index: cc/layers/picture_layer_impl.cc
diff --git a/cc/layers/picture_layer_impl.cc b/cc/layers/picture_layer_impl.cc
index fb8f70150878e07eaa6e356d4301e0cba92caf19..069ddb75b1859c9684f391b8f39614643f3749d0 100644
--- a/cc/layers/picture_layer_impl.cc
+++ b/cc/layers/picture_layer_impl.cc
@@ -943,12 +943,7 @@ void PictureLayerImpl::DoPostCommitInitialization() {
   if (!tilings_)
     tilings_.reset(new PictureLayerTilingSet(this, bounds()));
 
-  DCHECK(!twin_layer_);
-  twin_layer_ = static_cast<PictureLayerImpl*>(
-      layer_tree_impl()->FindActiveTreeLayerById(id()));
   if (twin_layer_) {
-    DCHECK(!twin_layer_->twin_layer_);
-    twin_layer_->twin_layer_ = this;
     // If the twin has never been pushed to, do not sync from it.
     // This can happen if this function is called during activation.
     if (!twin_layer_->needs_post_commit_initialization_)
@@ -958,6 +953,16 @@ void PictureLayerImpl::DoPostCommitInitialization() {
   needs_post_commit_initialization_ = false;
 }
 
+void PictureLayerImpl::UpdateTwinRelationship() {
+  DCHECK(!twin_layer_);
+  twin_layer_ = static_cast<PictureLayerImpl*>(
+      layer_tree_impl()->FindActiveTreeLayerById(id()));
+  if (twin_layer_) {
+    DCHECK(!twin_layer_->twin_layer_);
+    twin_layer_->twin_layer_ = this;
+  }
+}
+
 PictureLayerTiling* PictureLayerImpl::AddTiling(float contents_scale) {
   DCHECK(CanHaveTilingWithScale(contents_scale)) <<
       "contents_scale: " << contents_scale;