Add update observer to file_system_operation_impl_unittest for

content/public/test/sandbox_file_system_test_helper.cc

Index: content/public/test/sandbox_file_system_test_helper.cc
diff --git a/content/public/test/sandbox_file_system_test_helper.cc b/content/public/test/sandbox_file_system_test_helper.cc
index 55646c34a875387380748472fc4d0735ecea6ecb..38af9dfb644e05d5ae8190be8254d8c4e5d0245c 100644
--- a/content/public/test/sandbox_file_system_test_helper.cc
+++ b/content/public/test/sandbox_file_system_test_helper.cc
@@ -136,6 +136,12 @@ void SandboxFileSystemTestHelper::AddFileChangeObserver(
       type_, observer, NULL);
 }
 
+void SandboxFileSystemTestHelper::AddFileUpdateObserver(
+    storage::FileUpdateObserver* observer) {
+  file_system_context_->sandbox_delegate()->AddFileUpdateObserver(
+      type_, observer, NULL);
+}
+
 storage::FileSystemUsageCache* SandboxFileSystemTestHelper::usage_cache() {
   return file_system_context()->sandbox_delegate()->usage_cache();
 }