The code in gl::Texture would only check whether the first cube map face
is correctly initialized but not for the others.

BUG=angleproject:2169

Change-Id: Ib565579a4570e731da785b5c74c8d1a6511ebcd4
Reviewed-on: https://chromium-review.googlesource.com/960571
Commit-Queue: Corentin Wallez <cwallez@chromium.org>
Reviewed-by: Jamie Madill <jmadill@chromium.org>
Reviewed-by: Geoff Lang <geofflang@chromium.org>