Migrate //chromeos/ash/components/cros_disks to namespace ash [3/N]

This CL migrates enum chromeos::MountError to namespace ash and
changes to enum class.

This CL is part of the Chrome OS source code directory migration:
https://docs.google.com/document/d/1g-98HpzA8XcoGBWUv1gQNr4rbnD5yfvbtYZyPDDbkaE.

Change-Id: I836b54001bc5dfa3fe01a0f0782135c71cc675b0
Bug: 1164001
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/3802400
Reviewed-by: Hidehiko Abe <[email protected]>
Owners-Override: Hidehiko Abe <[email protected]>
Commit-Queue: Yeunjoo Choi <[email protected]>
Cr-Commit-Position: refs/heads/main@{#1030844}
diff --git a/ash/components/drivefs/drivefs_session.cc b/ash/components/drivefs/drivefs_session.cc
index 7e5aa9aa..a5553b1 100644
--- a/ash/components/drivefs/drivefs_session.cc
+++ b/ash/components/drivefs/drivefs_session.cc
@@ -55,11 +55,11 @@
 
  private:
   // MountPoint::Mount() done callback.
-  void OnMountDone(chromeos::MountError error_code,
+  void OnMountDone(ash::MountError error_code,
                    std::unique_ptr<ash::disks::MountPoint> mount_point) {
     DCHECK(callback_);
 
-    if (error_code != chromeos::MOUNT_ERROR_NONE) {
+    if (error_code != ash::MountError::kNone) {
       LOG(WARNING) << "DriveFs mount failed with error: " << error_code;
       std::move(callback_).Run({});
       return;