Activate LayerImpl tree with sync+push instead of pointer swap

cc/picture_pile.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 "cc/picture_pile.h"
 #include "cc/picture_pile_impl.h"
 #include "cc/region.h"
 
@@ skipping 20 matching lines @@
     return;
   }
 
   for (Region::Iterator i(invalidation); i.has_rect(); i.next())
     InvalidateRect(i.rect());
 
   for (Pile::iterator i = pile_.begin(); i != pile_.end(); ++i) {
     if (!(*i)->HasRecording())
       (*i)->Record(painter, stats);
   }
+  DCHECK(!pile_.empty());
 }
 
 class FullyContainedPredicate {
 public:
   FullyContainedPredicate(gfx::Rect rect) : layer_rect_(rect) { }
   bool operator()(const scoped_refptr<Picture>& picture) {
     return layer_rect_.Contains(picture->LayerRect());
   }
   gfx::Rect layer_rect_;
 };
@@ skipping 46 matching lines @@
 }
 
 void PicturePile::PushPropertiesTo(PicturePileImpl* other) {
   PicturePileBase::PushPropertiesTo(other);
 
   // Remove all old clones.
   other->clones_.clear();
 }
 
 }  // namespace cc