When mocking functions gmock uses the keyword virtual instead of
override and this conflicted with our usage of override for the
destructor (there is a warning for using all one or the other).

BUG=

Change-Id: I9dd9204d5755dd3a588a32d438de3c1018ba07cd
Reviewed-on: https://chromium-review.googlesource.com/291630Reviewed-by: Jamie Madill <jmadill@chromium.org>
Tested-by: Corentin Wallez <cwallez@chromium.org>