Add protobuf serialization to DisplayItemList

cc/playback/display_item.h

Index: cc/playback/display_item.h
diff --git a/cc/playback/display_item.h b/cc/playback/display_item.h
index bc089138ff9016b073ff8048dc22be631d3cd8cb..ed79eb0121ce16a12a2e841624a322d0e0075e90 100644
--- a/cc/playback/display_item.h
+++ b/cc/playback/display_item.h
@@ -16,6 +16,10 @@ class SkCanvas;
 
 namespace cc {
 
+namespace proto {
+class DisplayItem;
+}
+
 class CC_EXPORT DisplayItem {
  public:
   virtual ~DisplayItem() {}
@@ -28,6 +32,8 @@ class CC_EXPORT DisplayItem {
     external_memory_usage_ = external_memory_usage;
   }
 
+  virtual void ToProtobuf(proto::DisplayItem* proto) = 0;
+  virtual void FromProtobuf(const proto::DisplayItem& proto) = 0;
   virtual void Raster(SkCanvas* canvas,
                       const gfx::Rect& canvas_target_playback_rect,
                       SkPicture::AbortCallback* callback) const = 0;