Simplify directory path accounting.

webkit/fileapi/file_system_usage_cache.cc

 // Copyright (c) 2011 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 "webkit/fileapi/file_system_usage_cache.h"
 
 #include "base/file_path.h"
 #include "base/file_util.h"
 #include "base/pickle.h"
 
@@ skipping 56 matching lines @@
 }
 
 // static
 int FileSystemUsageCache::AtomicUpdateUsageByDelta(
     const FilePath& usage_file_path, int64 delta) {
   uint32 dirty = 0;
   int64 fs_usage;
   // TODO(dmikurube): Make sure that usage_file_path is available.
   fs_usage = Read(usage_file_path, &dirty);
 
+  if (fs_usage < 0)
+    return -1;
+
   return Write(usage_file_path, dirty, fs_usage + delta);
 }
 
 // static
 int FileSystemUsageCache::UpdateUsage(const FilePath& usage_file_path,
                                       int64 fs_usage) {
   return Write(usage_file_path, 0, fs_usage);
 }
 
 // static
@@ skipping 51 matching lines @@
   if (bytes_written != kUsageFileSize)
     return -1;
 
   if (file_util::ReplaceFile(temporary_usage_file_path, usage_file_path))
     return bytes_written;
   else
     return -1;
 }
 
 }