Some warnings were enabled only in standalone builds. Enabling them in
the GN config will prevent standalone build breakage and also make
some potentially harmful warnings more visible to non-Windows devs.

BUG=angleproject:1449

Change-Id: I11fe9bac957864d37af37b3860a22fcbd91946ac
Reviewed-on: https://chromium-review.googlesource.com/360961Reviewed-by: Geoff Lang <geofflang@chromium.org>
Reviewed-by: Olli Etuaho <oetuaho@nvidia.com>
Commit-Queue: Jamie Madill <jmadill@chromium.org>