cc/blimp: Add (de)-serialization for PictureLayer and ScrollbarLayer.

cc/layers/picture_layer.h

Index: cc/layers/picture_layer.h
diff --git a/cc/layers/picture_layer.h b/cc/layers/picture_layer.h
index 1aa5303c48dd83dbefc50d8d6cf4bb640d7af634..875588f9b4688efc5377d3adca582d0cb19b0ec9 100644
--- a/cc/layers/picture_layer.h
+++ b/cc/layers/picture_layer.h
@@ -68,6 +68,7 @@ class CC_EXPORT PictureLayer : public Layer {
   // Allow tests to inject a recording source.
   PictureLayer(ContentLayerClient* client,
                std::unique_ptr<RecordingSource> source);
+  PictureLayer(int engine_layer_id, ContentLayerClient* client);

[vmpstr, 2016/10/07 23:50:05]

This is... weird. Should this be "layer_id". I kind of don't like adding ctors
that are only used in tests. We usually use either ForTesting or overload for
this type of thing.

[Khushal, 2016/10/08 00:41:33]

Not needed anymore. Removed.

   ~PictureLayer() override;
 
   bool HasDrawableContent() const override;