Update conditions for exposing ASTC extensions. Add commenents to explain the Vulkan ASTC situation. Expose GL_KHR_texture_compression_astc_sliced_3d when the HDR profile exists. Bug: angleproject:4447 Change-Id: I00a86cb3c44873a28397aab33a47fb363d4ecfe9 Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/2112283 Reviewed-by: Alexis Hétu <sugoi@chromium.org> Reviewed-by: Alexey Knyazev <lexa.knyazev@gmail.com> Commit-Queue: Geoff Lang <geofflang@chromium.org>