Set root resource ID upon full feed update.

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 6c3439573491f5426d49bec6f4e8f5222fa1eb9f..83a6d81b22f2d197f3c6c3d00e18702186bfffbc 100644
--- a/chrome/browser/chromeos/drive/drive_resource_metadata.h
+++ b/chrome/browser/chromeos/drive/drive_resource_metadata.h
@@ -261,6 +261,10 @@ class DriveResourceMetadata {
                   const FileOperationCallback& callback);
   void SaveToDB();
 
+  // Returns the resource ID of the root directory or empty string if it is
+  // not initialized.
+  const std::string& root_resource_id() const { return root_resource_id_; }
+
  private:
   // Initializes the resource map using serialized_resources fetched from the
   // database.
@@ -306,6 +310,10 @@ class DriveResourceMetadata {
 
   scoped_ptr<DriveDirectory> root_;  // Stored in the serialized proto.
 
+  // Retains special resource ID for the root directory.
+  // This must be initialized only once using |InitializeRootEntry()|.
+  std::string root_resource_id_;
+
   base::Time last_serialized_;
   size_t serialized_size_;
   int64 largest_changestamp_;  // Stored in the serialized proto.