Make base:ReplaceFile return an informative error.

base/file_util.cc

Index: base/file_util.cc
diff --git a/base/file_util.cc b/base/file_util.cc
index e76c5c2fd9afc1958c991382e55e727bd0552726..1bed30f8d073229be00f30c688c31b7ee650ac0a 100644
--- a/base/file_util.cc
+++ b/base/file_util.cc
@@ -63,6 +63,11 @@ bool Move(const FilePath& from_path, const FilePath& to_path) {
   return MoveUnsafe(from_path, to_path);
 }
 
+bool ReplaceFile(const base::FilePath& from_path,
+                 const base::FilePath& to_path) {
+  return ReplaceFile(from_path, to_path, NULL);
+}
+
 bool CopyFile(const FilePath& from_path, const FilePath& to_path) {
   if (from_path.ReferencesParent() || to_path.ReferencesParent())
     return false;