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 c91e531d9d7e50cbb1710ab78d48f95678bf0a3e..2bc6c435d4ccb9a4e15360117356736151e82cc2 100644
--- a/cc/resources/picture_pile_impl.cc
+++ b/cc/resources/picture_pile_impl.cc
@@ -377,4 +377,22 @@ void PicturePileImpl::PixelRefIterator::AdvanceToPictureWithPixelRefs() {
   } while (AdvanceToTileWithPictures());
 }
 
+void PicturePileImpl::DidBeginTracing() {
+  gfx::Rect layer_rect(tiling_.total_size());
+  for (TilingData::Iterator tile_iter(&tiling_, layer_rect);

[enne (OOO), 2013/05/31 23:27:21]

It might be shorter to just iterate through the picture map and then each list
in the map.  No reason to use TilingData unless there's geometry involved.

+       tile_iter; ++tile_iter) {
+    PictureListMap::const_iterator map_iter =
+        picture_list_map_.find(tile_iter.index());
+    if (map_iter == picture_list_map_.end())
+      continue;
+    const PictureList& pic_list= map_iter->second;
+    if (pic_list.empty())
+      continue;
+    for (PictureList::const_reverse_iterator i = pic_list.rbegin();
+         i != pic_list.rend(); ++i) {
+      (*i)->DidBeginTracing();
+    }
+  }
+}
+
 }  // namespace cc