Add TRACE_EVENT_IS_NEW_TRACE as a way to snapshot objects at start of recording

cc/resources/picture_pile_impl.h

Index: cc/resources/picture_pile_impl.h
diff --git a/cc/resources/picture_pile_impl.h b/cc/resources/picture_pile_impl.h
index e5c03ea76f02ed4b3cbec9ee8a210fc716f4f65e..234a3907b9e44ac39f8c3fe793e904041342744e 100644
--- a/cc/resources/picture_pile_impl.h
+++ b/cc/resources/picture_pile_impl.h
@@ -109,6 +109,8 @@ class CC_EXPORT PicturePileImpl : public PicturePileBase {
     PictureList::const_iterator picture_list_iterator_;
   };
 
+  void DidBeginTracing();
+
  protected:
   friend class PicturePile;
   friend class PixelRefIterator;