Gmock defines virtual functions without the override keyword so the
constructors using override here caused a warning.

BUG=angleproject:892

Change-Id: I2a8f0445aa4a1e09694b7f98a6da07f8abaa47fb
Reviewed-on: https://chromium-review.googlesource.com/272551Reviewed-by: Jamie Madill <jmadill@chromium.org>
Tested-by: Corentin Wallez <cwallez@chromium.org>