Implement empty desctructor because of the forward declarations.

Change-Id: I56af92d7ded44b6fcd3582d1e3a10e9bb5cb1296
Reviewed-on: https://chromium-review.googlesource.com/905671Reviewed-by: Andreas Haas <ahaas@chromium.org>
Commit-Queue: Franziska Hinkelmann <franzih@chromium.org>
Cr-Commit-Position: refs/heads/master@{#51180}