Buffer11: Cleanup onCopyStorage. BUG=angleproject:1155 Change-Id: I7bc5c196d10ae040487ecd1137d9b18c3000775a Reviewed-on: https://chromium-review.googlesource.com/769730 Reviewed-by: Jamie Madill <jmadill@chromium.org> Reviewed-by: Yuly Novikov <ynovikov@chromium.org> Commit-Queue: Jamie Madill <jmadill@chromium.org>