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

cc/resources/picture_pile_impl.cc

 // Copyright 2012 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 #include <algorithm>
 #include <limits>
 
 #include "base/debug/trace_event.h"
 #include "cc/base/region.h"
 #include "cc/debug/debug_colors.h"
@@ skipping 358 matching lines @@
          ++picture_list_iterator_) {
       pixel_ref_iterator_ =
           Picture::PixelRefIterator(layer_rect_, picture_list_iterator_->get());
       if (pixel_ref_iterator_)
         return;
     }
     ++tile_iterator_;
   } 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