cc: Clear the |layer_id_to_scroll_offset_map_| during ScrollTree deserialization .

cc/trees/property_tree_unittest.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 "cc/trees/property_tree.h"
 
 #include "cc/input/main_thread_scrolling_reason.h"
 #include "cc/proto/property_tree.pb.h"
 #include "cc/test/geometry_test_utils.h"
 #include "cc/trees/draw_property_utils.h"
@@ skipping 263 matching lines @@
 
   original.set_currently_scrolling_node(1);
   original.SetScrollOffset(1, gfx::ScrollOffset(1, 2));
 
   proto::PropertyTree proto;
   original.ToProtobuf(&proto);
   ScrollTree result;
   result.FromProtobuf(proto);
 
   EXPECT_EQ(original, result);
+
+  original.clear();
+  original.set_currently_scrolling_node(0);
+  original.SetScrollOffset(2, gfx::ScrollOffset(1, 2));
+
+  proto::PropertyTree proto2;
+  original.ToProtobuf(&proto2);
+  result = ScrollTree();
+  result.FromProtobuf(proto2);
+
+  EXPECT_EQ(original, result);
 }
 
 TEST(PropertyTreeSerializationTest, PropertyTrees) {
   PropertyTrees original;
   original.transform_tree.Insert(TransformNode(), 0);
   original.transform_tree.Insert(TransformNode(), 1);
   original.clip_tree.Insert(ClipNode(), 0);
   original.clip_tree.Insert(ClipNode(), 1);
   original.effect_tree.Insert(EffectNode(), 0);
   original.effect_tree.Insert(EffectNode(), 1);
@@ skipping 791 matching lines @@
 
 DIRECT_AND_SERIALIZED_PROPERTY_TREE_TEST_F(
     PropertyTreeTestSingularTransformSnapTest);
 
 #undef DIRECT_AND_SERIALIZED_PROPERTY_TREE_TEST_F
 #undef SERIALIZED_PROPERTY_TREE_TEST_F
 #undef DIRECT_PROPERTY_TREE_TEST_F
 
 }  // namespace
 }  // namespace cc