Add protobuf serialization to DisplayItemList

cc/playback/compositing_display_item.h

Index: cc/playback/compositing_display_item.h
diff --git a/cc/playback/compositing_display_item.h b/cc/playback/compositing_display_item.h
index 9ee6b7a0eca70964d7ba85ee3f581a286fc91cb1..8cd9a76f403ca668e3ae17a33a6be77532d27825 100644
--- a/cc/playback/compositing_display_item.h
+++ b/cc/playback/compositing_display_item.h
@@ -28,6 +28,8 @@ class CC_EXPORT CompositingDisplayItem : public DisplayItem {
               SkRect* bounds,
               skia::RefPtr<SkColorFilter> color_filter);
 
+  void ToProtobuf(proto::DisplayItem* proto) override;
+  void FromProtobuf(const proto::DisplayItem& proto) override;
   void Raster(SkCanvas* canvas,
               const gfx::Rect& canvas_target_playback_rect,
               SkPicture::AbortCallback* callback) const override;
@@ -52,6 +54,8 @@ class CC_EXPORT EndCompositingDisplayItem : public DisplayItem {
     return make_scoped_ptr(new EndCompositingDisplayItem());
   }
 
+  void ToProtobuf(proto::DisplayItem* proto) override;
+  void FromProtobuf(const proto::DisplayItem& proto) override;
   void Raster(SkCanvas* canvas,
               const gfx::Rect& canvas_target_playback_rect,
               SkPicture::AbortCallback* callback) const override;