Load the sampler object entry points and limit supported ES version to 2
if samplers are not available.

BUG=angleproject:1162

Change-Id: Ida7cf0cc0a81dd3714d958a98efe0c6e973ecca2
Reviewed-on: https://chromium-review.googlesource.com/303770Reviewed-by: Corentin Wallez <cwallez@chromium.org>
Tested-by: Geoff Lang <geofflang@chromium.org>