Unset version requirements equate to UINT_MAX causing requirements checks
for fail for formats that only need extensions such as DXT formats.

Fixes tests that require compressed formats.

Passes conformance/extensions/webgl-compressed-texture-s3tc.html

BUG=angleproject:884

Change-Id: Id810c477a359bf9d35a886c60735a91b51d1e931
Reviewed-on: https://chromium-review.googlesource.com/275693Reviewed-by: Jamie Madill <jmadill@chromium.org>
Reviewed-by: Corentin Wallez <cwallez@chromium.org>
Tested-by: Geoff Lang <geofflang@chromium.org>