content/blimp: Set up hooks for enabling LTHRemote in the renderer.

cc/layers/picture_layer.cc

Index: cc/layers/picture_layer.cc
diff --git a/cc/layers/picture_layer.cc b/cc/layers/picture_layer.cc
index 408051bb218b571e4909ca3e424c3d2995cc0cb3..a435e68ee2c0fa4619a48cd58f6f249f2a747b39 100644
--- a/cc/layers/picture_layer.cc
+++ b/cc/layers/picture_layer.cc
@@ -208,6 +208,9 @@ void PictureLayer::SetTypeForProtoSerialization(proto::LayerNode* proto) const {
 
 void PictureLayer::LayerSpecificPropertiesToProto(proto::LayerProperties* proto,
                                                   bool inputs_only) {
+  DCHECK(GetLayerTree());
+  DCHECK(GetLayerTree()->engine_picture_cache());
+
   Layer::LayerSpecificPropertiesToProto(proto, inputs_only);
   DropRecordingSourceContentIfInvalid();
   proto::PictureLayerProperties* picture = proto->mutable_picture();