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

cc/resources/picture_pile_impl.cc

Index: cc/resources/picture_pile_impl.cc
diff --git a/cc/resources/picture_pile_impl.cc b/cc/resources/picture_pile_impl.cc
index d992af1596b0478de96d720afe773dcf604b1623..46aa79f169f1f5b56d5f0a469d641fc03e2d67dc 100644
--- a/cc/resources/picture_pile_impl.cc
+++ b/cc/resources/picture_pile_impl.cc
@@ -376,4 +376,18 @@ void PicturePileImpl::PixelRefIterator::AdvanceToPictureWithPixelRefs() {
   } while (AdvanceToTileWithPictures());
 }
 
+void PicturePileImpl::DidBeginTracing() {
+  gfx::Rect layer_rect(tiling_.total_size());
+  for (PictureListMap::iterator pli = picture_list_map_.begin();
+       pli != picture_list_map_.end();
+       pli++) {
+    PictureList& picture_list = (*pli).second;
+    for (PictureList::iterator picture = picture_list.begin();
+         picture != picture_list.end();
+         picture++) {
+      (*picture)->EmitTraceSnapshot();
+    }
+  }
+}
+
 }  // namespace cc