Fix warnings caused by gmock on linux 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/291630 Reviewed-by: Jamie Madill <jmadill@chromium.org> Tested-by: Corentin Wallez <cwallez@chromium.org>