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 195da00228ce673f454d85fb6c67824d93ac4f9e..5f4004b7d9d7af7b84093e07d7c419ae4a246bac 100644
--- a/chrome/browser/chromeos/drive/drive_resource_metadata.h
+++ b/chrome/browser/chromeos/drive/drive_resource_metadata.h
@@ -59,10 +59,6 @@ enum ContentOrigin {
 // name is used in URLs for the file manager, hence user-visible.
 const FilePath::CharType kDriveRootDirectory[] = FILE_PATH_LITERAL("drive");
 
-// The resource ID for the root directory is defined in the spec:
-// https://developers.google.com/google-apps/documents-list/
-const char kDriveRootDirectoryResourceId[] = "folder:root";
-
 // This should be incremented when incompatibility change is made in
 // drive.proto.
 const int32 kProtoVersion = 2;
@@ -262,6 +258,10 @@ 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;
+
  private:
   // Initializes the resource map using serialized_resources fetched from the
   // database.
@@ -307,6 +307,11 @@ class DriveResourceMetadata {
 
   scoped_ptr<DriveDirectory> root_;  // Stored in the serialized proto.
 
+  // Retains special resource ID for the root directory across |ClearRoot()|.
+  // This must be initialized only once using |InitializeRootEntry()|.
+  // This value will be used to look up the root direcotry in |InitFromDB()|.
+  std::string saved_root_resource_id_;
+
   base::Time last_serialized_;
   size_t serialized_size_;
   int64 largest_changestamp_;  // Stored in the serialized proto.