This was generating a warning for missing virtual destructor in
some configurations.

BUG=angleproject:2167

Change-Id: I05e335de4ab1160c6066665d588b50527ea34cbd
Reviewed-on: https://chromium-review.googlesource.com/723980Reviewed-by: Jamie Madill <jmadill@chromium.org>
Commit-Queue: Jamie Madill <jmadill@chromium.org>