Remove root resource id aliasing from DriveResourceMetadata.

chrome/browser/chromeos/drive/drive_resource_metadata.h

Index: chrome/browser/chromeos/drive/drive_resource_metadata.h
diff --git a/chrome/browser/chromeos/drive/drive_resource_metadata.h b/chrome/browser/chromeos/drive/drive_resource_metadata.h
index b796db824f36b29dfee9c0118d01932e51af8f50..dca0bd4ae13a67ea3bee3431bbfb70c675f4d239 100644
--- a/chrome/browser/chromeos/drive/drive_resource_metadata.h
+++ b/chrome/browser/chromeos/drive/drive_resource_metadata.h
@@ -134,7 +134,6 @@ class DriveResourceMetadata {
  public:
   // |root_resource_id| is the resource id for the root directory.
   DriveResourceMetadata(
-      const std::string& root_resource_id,
       const base::FilePath& data_directory_path,
       scoped_refptr<base::SequencedTaskRunner> blocking_task_runner);
 
@@ -393,8 +392,6 @@ class DriveResourceMetadata {
 
   scoped_ptr<DriveResourceMetadataStorage> storage_;
 
-  std::string root_resource_id_;
-
   base::Time last_serialized_;
   size_t serialized_size_;