cc: Clear the |layer_id_to_scroll_offset_map_| during ScrollTree deserialization .

cc/trees/property_tree.cc

 // Copyright 2014 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 <stddef.h>
 
 #include <set>
 #include <vector>
 
 #include "base/logging.h"
@@ skipping 1444 matching lines @@
 
 void ScrollTree::FromProtobuf(const proto::PropertyTree& proto) {
   DCHECK(proto.has_property_type());
   DCHECK_EQ(proto.property_type(), proto::PropertyTree::Scroll);
 
   PropertyTree::FromProtobuf(proto);
   const proto::ScrollTreeData& data = proto.scroll_tree_data();
 
   currently_scrolling_node_id_ = data.currently_scrolling_node_id();
 
+  // TODO(khushalsagar): This should probably be removed if the copy constructor
+  // for ScrollTree copies the |layer_id_to_scroll_offset_map_| as well.
+  layer_id_to_scroll_offset_map_.clear();
   for (int i = 0; i < data.layer_id_to_scroll_offset_map_size(); ++i) {
     const proto::ScrollOffsetMapEntry entry =
         data.layer_id_to_scroll_offset_map(i);
     layer_id_to_scroll_offset_map_[entry.layer_id()] = new SyncedScrollOffset();
     ProtoToSyncedScrollOffset(
         entry.scroll_offset(),
         layer_id_to_scroll_offset_map_[entry.layer_id()].get());
   }
 }
 
@@ skipping 463 matching lines @@
       break;
     case ALL_TREES:
       transform_tree.ResetChangeTracking();
       effect_tree.ResetChangeTracking();
   }
   changed = false;
   full_tree_damaged = false;
 }
 
 }  // namespace cc