Add move constructor to LinkNode.

This makes it possible to use LinkNode subclasses with base::Optional.

Bug: 729800
Change-Id: If2f3814879ec252ea0723f5ad1070d5917445e34
Reviewed-on: https://chromium-review.googlesource.com/1005022
Commit-Queue: Victor Costan <pwnall@chromium.org>
Reviewed-by: Nico Weber <thakis@chromium.org>
Cr-Commit-Position: refs/heads/master@{#549565}
2 files changed