wstring: use a few more FilePaths to remove Hack functions

chrome/installer/util/copy_tree_work_item.h

Index: chrome/installer/util/copy_tree_work_item.h
diff --git a/chrome/installer/util/copy_tree_work_item.h b/chrome/installer/util/copy_tree_work_item.h
index 2c7723700e15f27df1f63ad9d257cf4b497a8274..830a0d4d6bdba126b8bfbb4414b9982b0b19a54f 100644
--- a/chrome/installer/util/copy_tree_work_item.h
+++ b/chrome/installer/util/copy_tree_work_item.h
@@ -37,11 +37,11 @@ class CopyTreeWorkItem : public WorkItem {
   // Notes on temp_path: to facilitate rollback, the caller needs to supply
   // a temporary directory to save the original files if they exist under
   // dest_path.
-  CopyTreeWorkItem(const std::wstring& source_path,
-                   const std::wstring& dest_path,
-                   const std::wstring& temp_dir,
+  CopyTreeWorkItem(const FilePath& source_path,
+                   const FilePath& dest_path,
+                   const FilePath& temp_dir,
                    CopyOverWriteOption overwrite_option,
-                   const std::wstring& alternative_path);
+                   const FilePath& alternative_path);
 
   // Checks if the path specified is in use (and hence can not be deleted)
   bool IsFileInUse(const FilePath& path);