Simplify directory path accounting.

webkit/fileapi/file_system_usage_cache.cc

Index: webkit/fileapi/file_system_usage_cache.cc
diff --git a/webkit/fileapi/file_system_usage_cache.cc b/webkit/fileapi/file_system_usage_cache.cc
index eee383f4a533a48353322a7419f8b560b9f2eb87..6fa788e8fbcc5e6f931c3d28dfb965d1b71ae467 100644
--- a/webkit/fileapi/file_system_usage_cache.cc
+++ b/webkit/fileapi/file_system_usage_cache.cc
@@ -74,6 +74,9 @@ int FileSystemUsageCache::AtomicUpdateUsageByDelta(
   // 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);
 }
 
@@ -144,4 +147,4 @@ int FileSystemUsageCache::Write(const FilePath& usage_file_path,
     return -1;
 }
 
-}
+}  // namespace fileapi