Don't clobber active page scale animation with null pointer.

cc/trees/layer_tree_host_impl.cc

 // Copyright 2011 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 "cc/trees/layer_tree_host_impl.h"
 
 #include <algorithm>
 #include <limits>
 
 #include "base/basictypes.h"
@@ skipping 1800 matching lines @@
     const RenderingStats& stats =
         rendering_stats_instrumentation_->GetRenderingStats();
     paint_time_counter_->SavePaintTime(stats.main_stats.paint_time +
                                        stats.main_stats.record_time +
                                        stats.impl_stats.rasterize_time);
   }
 
   if (time_source_client_adapter_ && time_source_client_adapter_->Active())
     DCHECK(active_tree_->root_layer());
 
-  page_scale_animation_ = active_tree_->TakePageScaleAnimation();
-  if (page_scale_animation_) {
+  scoped_ptr<PageScaleAnimation> page_scale_animation =
+      active_tree_->TakePageScaleAnimation();
+  if (page_scale_animation) {
+    page_scale_animation_ = page_scale_animation.Pass();
     SetNeedsAnimate();
     client_->SetNeedsCommitOnImplThread();
     client_->RenewTreePriority();
   }
 }
 
 void LayerTreeHostImpl::SetVisible(bool visible) {
   DCHECK(proxy_->IsImplThread());
 
   if (visible_ == visible)
@@ skipping 1552 matching lines @@
 }
 
 void LayerTreeHostImpl::UnregisterPictureLayerImpl(PictureLayerImpl* layer) {
   std::vector<PictureLayerImpl*>::iterator it =
       std::find(picture_layers_.begin(), picture_layers_.end(), layer);
   DCHECK(it != picture_layers_.end());
   picture_layers_.erase(it);
 }
 
 }  // namespace cc