Enable more warnings on GCC and clang BUG=angleproject:892 Change-Id: I74ca341f29b245f698d1e1ad43149a91db46817f Reviewed-on: https://chromium-review.googlesource.com/271411 Reviewed-by: Jamie Madill <jmadill@chromium.org> Reviewed-by: Geoff Lang <geofflang@chromium.org> Tested-by: Corentin Wallez <cwallez@chromium.org>