cc: Clear the |layer_id_to_scroll_offset_map_| during ScrollTree deserialization .

cc/trees/property_tree_unittest.cc

Index: cc/trees/property_tree_unittest.cc
diff --git a/cc/trees/property_tree_unittest.cc b/cc/trees/property_tree_unittest.cc
index 6cc1cf148e613f144911c76c60b9a76fee930ae0..33bb1f7b801b4dc3622f136a3037ee413a2b3f38 100644
--- a/cc/trees/property_tree_unittest.cc
+++ b/cc/trees/property_tree_unittest.cc
@@ -281,6 +281,17 @@ TEST(PropertyTreeSerializationTest, ScrollTreeSerialization) {
   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) {