We should just call Attr::Clone().

Bug: 812089, 812105
Change-Id: I163769e134a52d82b88834ae29083a01a3480e95
Reviewed-on: https://chromium-review.googlesource.com/925865


Reviewed-by: Yoshifumi Inoue <yosin@chromium.org>
Commit-Queue: Kent Tamura <tkent@chromium.org>
Cr-Commit-Position: refs/heads/master@{#537730}